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# GMAT: Math 30

154-160, pg. 173. Created by Sal Khan.

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• I am confused here, could anyone help, please. It said "the tank contains 10,000 gallons of a SOLUTION", as I understand it, this SOLUTION is a combination of WATER + SALT. And as the salt is 500 gallons, it makes the water 9,500 gallons. If 2,500 gallons of water evaporates, the remaining water will be 9,500 - 2,500 = 7,000 gallons? Which will give a much different answer.
Could anyone please explain? Thank you so much
• 154. what did i do wrong? i was okay up to this point.
i got 40/3 = 5x/2,
cross multiply to come up with 20/3 = 5x,
x = 4/3.
x=1 1/3
(1 vote)
• 40/3 = 5x/2
Cross Multiply and then you come up with:
40*2 = 5x*3
And then:
80 = 15x
You divide 15 to both sides:
80/15 = x
5.3333... = x
Which is:
5 and 1/3
• 154. 8/3 = x/2
why cant i now cross multiply to come up with 4/3 = x?
(1 vote)
• 8/3 = x/2
You cross multiply:
8*2 = 3*x
Which is equal to:
16 = 3x
Divide both sides by 3:
5.3333 = x
Which is equal to:
5 and 1/3 = x
• 154. why cant i say : 8/.6 = x/.4 then cross multiply .6x=.32 then divide and get 5.3 which is 5 hours and 30 minutes ?
• 154. I got a alternate way to solve it with a similar reasoning.

In 10000 Liters 5% is Sodium Chloride ie. 500 kgs
2500 liters evaporate so we are left with 7500 liters. Also we know the salt doesn't change

So 500 = x% of 7500 [500 is the salt which stays the same even after the water evaporates]

500 = x/100 x 7500
x=500x100 /7500
= 6.6666 or 6.67%
(1 vote)
• 155. At , could I divide both sides by X instead of 3? Then I wouldn't have to move the three back over?
(1 vote)
• 160. Another easier option with less Math involved.

There are two circles as in the figure and the Radius of the larger one is *(8+3) while that of the smaller one is 3. We need to find the area of circular path.

R1 = 11 and r2 = 8

Set the formula as the
Area = pie (R square - r square)
= pie (11 square - 8 square)

remember a square - b square = (a+b) (a-b)
so the equation above is pie times (11+8) (11-8) = pie (19)(3) = 57 pie

saves us a lot of multiplication and squares if we use this formula.