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## GMAT

### Course: GMAT > Unit 1

Lesson 1: Problem solving- GMAT: Math 1
- GMAT: Math 2
- GMAT: Math 3
- GMAT: Math 4
- GMAT: Math 5
- GMAT: Math 6
- GMAT: Math 7
- GMAT: Math 8
- GMAT: Math 9
- GMAT: Math 10
- GMAT: Math 11
- GMAT: Math 12
- GMAT: Math 13
- GMAT: Math 14
- GMAT: Math 15
- GMAT: Math 16
- GMAT: Math 17
- GMAT: Math 18
- GMAT: Math 19
- GMAT: Math 20
- GMAT: Math 21
- GMAT: Math 22
- GMAT: Math 23
- GMAT: Math 24
- GMAT: Math 25
- GMAT: Math 26
- GMAT: Math 27
- GMAT: Math 28
- GMAT: Math 29
- GMAT: Math 30
- GMAT: Math 31
- GMAT: Math 32
- GMAT: Math 33
- GMAT: Math 34
- GMAT: Math 35
- GMAT: Math 36
- GMAT: Math 37
- GMAT: Math 38
- GMAT: Math 39
- GMAT: Math 40
- GMAT: Math 41
- GMAT: Math 42
- GMAT: Math 43
- GMAT: Math 44
- GMAT: Math 45
- GMAT: Math 46
- GMAT: Math 47
- GMAT: Math 48
- GMAT: Math 49
- GMAT: Math 50
- GMAT: Math 51
- GMAT: Math 52
- GMAT: Math 53
- GMAT: Math 54

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# GMAT: Math 14

76-80, pg. 162. Created by Sal Khan.

## Want to join the conversation?

- on question 76, when you reach 3.2/2 if you change it to 32/20 and do you division you will end up with 8/5 which is A, just seems shorter this way, right?(6 votes)
- #78: I don't understand why this doesn't work: 45*.85. The answer is 38.25 which is incorrect, but can someone explain the logic behind why it doesn't work?(1 vote)
- This might get lengthy.

When you are doing 45*85 --> you are calculating a 15% reduction on the 45.

Let me give you an example:

An item costs 100$ --> It increases by 20% to become 120$. --> After the increment there is a discount and the (increased) price (120) is reduced by 20%.

20% of 120 = 24. Therefore reduction of 20% = 120-24 =96.

Which is equal to 120*80% = 96.

But since the original increase of 20% was applied to base of 100$ and the decrease of 20% is applied to 120$ the values of the 20%s are not equal.

In the same way, if you reduce 15% on the (already increased) 45m, it will not be the same amount as the original 15% increment.

Hope this clears your doubt, let me know if there are any more questions.(6 votes)

- How will this help me if I am in the 1st grade(2 votes)
- for number 79. where does the 2 come from? that is not a square root of anything....shouldnt it be 1, 4, 9, etc....(1 vote)
- Bigger math = calculus? Well I dont know if it doesnt make sense grammatically or perhaps I just cant decipher what your answer is, but could you rephrase your answer?(3 votes)

- #78 another way.. 1.15* 38= 43 no1.15* 39= 44.85 yes( the nearst one 45 m)

To be sure you can multiply another numbers by 1.15(0 votes) - For #79, where was he getting those numbers (1, 2, 4, 9, 16, 25, 36, 49)?(0 votes)
- Im thinking he meant to use 3 and not 2. @8:09he references the number 3 in his assessment of the number 9. 2 is just a typo.(1 vote)

- in the beginning of question 78 why did you put an x?(im 12 years old,im studying this)(0 votes)
- #78, shorter way is to multiply 45M by .15, then subtract from 45M

ex: 45M x .15 = 6.75 45M - 6.75M = 39.25M (answer B)(0 votes) - QUESTION 78 WHERE IS THE ONE COMING X + .15X = 45

1.15X = 45

WHERE DOES THE 1.15X COME FROM(0 votes)- Remember that "x" is just shorthand for "one lot of x", "1 times x" or "1x". So x + 0.15x = 1x + 0.15x = (1+0.15)x = 1.15x. Hope that helps.(3 votes)

## Video transcript

So we're at problem 76. Which of the following ratios
is most nearly equal to the ratio of 1 plus the square
root of 5 to 2. And then they give us a bunch of
integer choices, so this is really just a problem of
approximating what the square root of 5 is. So let's think about
this a little bit. So 2 is equal to the square
root of 4, or 2 squared is equal to 4. 3 is equal to the square
root of 9. So the square root of 5 is going
to be a lot closer to 2. What's 2.1 squared? 2.1 times 2.1 you get 21. 0, 2 times 21 is 42. So then you get, 4.41
So 2.1 is equal to square root of 4.41. 2.2 times 2.2, 2 is 44, 0, 44,
this gets pretty close to 5. 8, 4. So 2.2 is I think about as close
as we're going to get. If we approximate the square
root of 5 is 2.2, and this is just an approximation, we just
have to figure out which choice is closest, 1 plus 2.2
over 2, that's equal to 3.2 over 2, and that's not one of
the choices, because this really isn't an integer
choice. But let's see if I can
just express this, this is equal to 1.6. It'd give you 1.6:1. So we just have to find which
of the choices are roughly equal to 1.6:1. So choice A, 8:5. Well what is that? That's 1 and 3/5. Well that's exactly 1.6. So that's the choice I'm
going to go with. Let me look at the other ones. 6:5 is going to be a lot closer
to 1, 6:5 is 1.2, 5:4 is 1.25, 2:1 is 2. So choice A is the
best choice. Next question. Problem 77. Let me scroll down
a little bit. 7 over 1/5 plus 5 over 1/7 is
equal to-- 7 divided by 1/5 is equal to 7 times 5, plus
5 divided by 1/7 is equal to 5 times 7. When you divide by a fraction,
it's the same thing as multiplying by its reciprocal. So 5 times 7. So this is equal to 35 plus 35,
which is equal to 70, and that's choice D. 78. From January 1, 1991, to January
1, 1993, the number of people enrolled in health
maintenance organizations increased by 15%. The enrollment on January
1, 1993 was 45 million. How many million people, to
the nearest million, were rolled in health maintenance
organizations on January 1, 1991? OK, so let's just call this x. That's what they want to know,
they want to know how many million people were
enrolled in 1991. So when x increased by 15%,
you got to 45 million. So remember, not 15% of
x is 45 million, x increases by 15%. So let me write an extra step
that I normally don't do. So you could view it as
x plus 15% x, right? This is x, and then you're
increasing it by 15% of x, is going to be equal
to 45 million. And then this is 1x plus 0.15x,
so that's 1.15x is equal to 45 million. And you could also just go
straight to this step. In fact, you usually can once
you get used to it. You can say, 15% increase,
that's the same thing as multiplying by 1.15. So x is equal to 45 divided by
1.15, and we're going to have to do a little bit of division
to figure this out. 115 divided by 45, add a couple
of 0's So it's 1.15 divide by 45, so that's the same
thing as 115, take the decimal place, go 2 to the
right, going to 4,500. 115 goes into 450
how many times? Does it go into it 4 times? No, that'd be 460, so it
goes into it 3 times. 3 times 115 is 345. This difference is 105,
bring down a 0. 115 goes into 1,050
how many times? If I did 8, 800, I think it's 8
times, because 9 would be-- actually 900 plus 90 plus 45,
actually it would be 9 times. 9 times 5 is 45. 9 times 1 is 9 plus 4 is 13. 9 times 9 is 1-- I'm sorry,
is 9, plus 1 is 10. You have a remainder 15, bring
down the 0, that goes 1 time. So we already have the
decimal point. And all their choices
are in integers. So this is closest to B, 39. And I wanted to get that last
decimal point, because maybe that was 39.9, in which
case 40 would be the closest choice. Sometimes they get
tricky like that. Next question. 79. R is the set of positive odd
integers less than 50. And S is equal to squares
of R, squares of the numbers in that set. How many elements does the intersection of R and S contain? OK, so we have to say,
positive odd editors less than 50. So we have to think, how many
positive odd integers less than 50 are squares of
other positive odd integers less than 50. It doesn't take a long time to
figure out all of the squares between 1 and 50. It's 1, 2, 4, 9, 16, 25,
they start increasing pretty fast, 36, 49. Now we know that they
have to be positive, these are all positive. We know they have to be odd,
because we're looking for things that are both squares
and they're in set R. They're squares, and they're
squares of other things in set R. So let's take out all
the even numbers. Can't be 36, can't be 2, can't
be 4, can't be 16. 1 is 1 squared. So 1 is in set R, it's a
positive odd integer less than 50, and it is a square of
another positive odd integer less than 50, a square
of itself. So 1 counts. 9 is 3 squared. 9 itself is a positive odd
integer less than 50, and it's also the square of 3, which is
another positive odd integer less than 50. So 9 counts. 25, same logic. It's a positive odd integer less
than 50, it's the square of 5, which is the same. And 49, same thing. Square of 7, both 49 and
7 are positive odd integers less than 50. So I found four. So that's C. Question 80. A retail appliance store priced
a video recorder at 20% above the wholesale
cost of $200. We could just go, 20% above
200, that's going to be 1.2 times 200. If a store employee applied the
10% employee discount to the retail price to buy the
recorder, how much did the employee pay for the recorder? So this is the retail price. This was the wholesale price,
that's what the store bought it for, this is what a
non-employee would have to pay, 1.2 times 200. And then the employee
got a 10% discount. So essentially the
employee paid the retail price times 0.9. So what's 1.2 times 200? 200 times 1.2. Let's just do it out. 2 times 200 is 400. Add a zero. 1 times 200 is 200. And we have one point behind the
decimal-- well let's add them up, you get 2400. We have one digit behind the
decimal, so it's 240. So the retail price is 240, and
we have to multiply that times 0.9 to figure out what
the employee paid. They got a 10% discount, which
is another way of saying they played 90% of the
retail price. So 240 times 0.9. 9 times 0 is zero, 9 times
4 is 36, 9 times 2 is 18 plus 3, 21. We have one number behind the
decimal point, so the employee paid $216, which is choice B. Now I'm out of time,
I'll continue this in the next video.