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# GMAT: Data sufficiency 33

Video transcript

We're on problem 132. If the integer n is greater
than 1, is n equal to 2? So they tell us that the integer
n is greater than 1, and they ask us, is
n equal to 2? Statement 1, n has exactly
two positive factors. Well that's certainly true
of the number 2. But it's also true of
any prime number. I mean n could be 7. 7 only has two positive
factors, 1 and 7. So it's true of any
prime number. So this, by itself, isn't enough
information to say that it's definitely 2. Statement 2 tells us the
difference of any two distinct positive factors of n is odd. This is interesting. We're just dealing in kind
of the positive world. So let's think about it. Just thinking about statement
1, we said, OK, what numbers have only two factors? Well prime numbers only
have two factors. This only tells us
that n is prime. 2 is a prime number, but it's
not the only prime number. But for most other prime
numbers, what do we know about them? Well they're odd, right? Most prime numbers are odd. Other than 2, what are the
other prime numbers? You go to 3, you go to
7, you go to 11, 5. Let me ask you, why
are they odd? Because if they were even, they
would be divisible by 2 and they wouldn't be prime. So by definition really, every
prime number that's not 2 is not divisible by 2 because
it's prime. So they have to be
odd numbers. So for any other prime number,
the number is going to be 1 and itself. So let's call that p. So the difference between the
two numbers, and p is going to be an odd number. Every other prime number
other than 2 is odd. So the difference between
the two, p minus 1. If I take an odd number and I
subtract one from it, I get an even number. That's true for any
odd number. So every other prime
number is odd. And this happens. But they're saying that when I
subtract the difference I get an odd, I get an odd number. Well the only number that's
true for is 2. Because factors of
2 are 1 and 2. If I subtract 1 from 2, 2
minus 1 is equal to 1. That's because 2 is the only
even prime number. So this first statement says we
have to be dealing with a prime number. The second statement says, well,
the number really has to be an even number if it's
going to be prime. Let me think about the
other two-- if statement 2 alone is enough. The difference of any two
positive factors of n is odd, any two positive factors of n. Now this alone doesn't help us
because n could be-- I don't know-- it could be 1 and 6. The difference between
1 and 6 is 5, so it would satisfy this. So n could be 6 if we just
took statement 2. So we really need both. Statement 1 tells us
that n is prime. Statement 2 tells us that n has
to be even, that it has to be an even number. So frankly, there's only
one prime even number, and that's 2. So both statements together
are necessary to answer this question. 133, every member of a certain
club volunteers to contribute equally to the purchase price
of a $60 gift certificate. How many members of
does a club have? So we want to know-- let's call
m for members-- how many members does a club have? Question 1, each member's
contribution is to be $4. Let's see, it says they
contribute equally to a purchase price. OK, so the number of members
times-- and they say every member of a certain club. So now they don't say
some members. So m is the number of
members, and they contribute equally $4. That's going to be
equal to $60. So then you immediately know
that you can solve for the number of members. There are 15 members
of this club. Maybe I'm missing something. Statement 2, if 5 club members
fail to contribute, the share of each contributing member
will increase by $2. So that means that if I were
to take the amount that 5 members were to contribute--
so let's say that the contribution amount is c. So if we take the amount that 5
members would contribute, so 5 times c, and divide it by
the remaining members, so divide by m minus 5, that
would be that each contributing member will
increase by $2. So this is the amount that would
have been contributed by those 5 people. Let me make sure that I'm
not missing anything. That is going to
be equal to $2. So this is the amount that those
5 members would have contributed. If they don't contribute it,
it's going to have to be divided by the other members. So however many members
there are minus 5. That is going to be-- when you
divide the amount divided by who has to pay for it-- is
$2 per leftover member. We also know that the members
times the contribution for member if everyone pays, is
going to be equal to $60. That they give us in the problem
statement, that m members are going to contribute
equally, and they're going to end
up with $60. So actually, we have two linear equations and two unknowns. So statement 2 alone is actually
sufficient to solve this problem. If this doesn't look like a
linear question, you can just multiply both sides by m minus
5 and you get 5c is equal to 2m minus 10. Now this looks a lot more
like a linear equation. Well, actually, this isn't a
complete linear equation. But let me solve it just to make
the point clear for you. So if I were to say that c--
this isn't a linear equation, so I shouldn't have said that--
c is equal to 60/m. If c is equal to 60/m, so then
this turns to 5 times 60. So 300/m is equal
to 2m minus 10. Then you are left with what? Multiply both sides by m, you
get 300 is equal to 2m squared minus 10m. Divide both sides by 2, you
get 150 is equal to m squared minus 10m. Then subtract 150
from both sides. You get m squared minus 10m
minus 150 is equal to 0. Then let me see if I can just do
this just by factoring it. Minus 150, 30 times 5,
no, that's not good. 15 times 10. So if I do m minus 15 times--
no, that doesn't work. 15, 25, and 6. I know one of the
answers already. I know it's 15. m minus 15 times m plus 10. Oh, actually, I just realized
what my mistake was. I went from this step
to this step. So I divided both sides by 2. So 300 went into 150. 2m squared-- this
had to be 5m. That's my mistake. 5m. M squared minus 5m minus 150. So that's m minus 15 times
m plus 10 is equal to 0. So that tells us that m is
equal to 15 or minus 10. This is actually a quadratic
equation. We know that the members
can't be negative. There's a positive number
of members. So statement 2 alone is enough
information to know that there are exactly 15 members
in this club. Next problem. It had me stumped there because
of my careless mistake there for a second. Next problem. OK, problem 134. So that last one, I don't
know if I just said it. Each statement alone
is sufficient. So problem 134. If m and n are positive
integers, is the square root of n minus m an integer? So m and n are positive
integers. OK, so they tell us. Statement 1, they say n is
greater than m plus 15. Well you know, n is greater
than m plus 15. So if n is exactly 16 greater
than m, this is another way of saying that n minus m
is greater than 15. That tells that what's under
the denominator is greater than 15. Well, if what's under the
denominator is 16, then it is an integer. But if it's 17, which also meets
this requirement, it's not an integer. So this isn't enough information
by itself. Statement 2 says n is equal to m
times m plus 1, which is the same thing as m squared
plus m. So if we substitute that into
this equation, we get the square root of m squared
plus m minus m. That's m times m plus 1. It's m squared plus m. This minus m is right there. So that cancels out and we're
just left with square root of m squared, which is going
to be equal to m, which is an integer. So statement 2 alone is
sufficient to say that this would be an integer as long
as n is equal to m times m plus 1. I've run out of time. See you in the next video.