Expert: Sherman D. - 9/29/2004

Question
I'm having touble understanding how my book got their equation to solve an example.Here is the example:
How many liters of water must be evaporated from 84 liters of a 20% salt solution to raise it to a 35% salt solution?My book gives the proportion, (0.20*84)/(84-x)=(35/100).Please explain how they got that.And if there are easier ways to solve this, please tell me as well. Thank you.

Answer
84 liters of 20% means 84 * 20% or 84 * .20

so that gives you the (.20 * 84) part

as for 84 - x, when it says evaporate it means how much water will be taken out of 84, so that gives you the (84 - x) part

as for the (35/100) that is how much percentage of salt solution will it become when you evaporate the water.

To find the percentage, you will have to take the average of salt solution per liter, so this gives you

(.2 * 84)/(84 - x) = (35/100)

becomes

(16.8)/(84 - x) = (35/100)

cross multiply and you get

1680 = 2940 - 35x

-35x = -1260

x = 36 liters