Solution Percents

Question

How many liters of a 30% alcohol solution must be mixed with 90 liters of a 90% solution to get a 70% solution?

Answer 1

Let there be x liters of 30% alcohol.

(0.3x+90*0.9) / (x+90) = 0.7

0.3x + 81 = 0.7x+63

18 = 0.4x

x =45

 

Answer 2

For simplicity, lets convert % to decimals, then assuming the unknown variable X represents the number of liters of 30% alcohol solution, write this simple math equation:

 

(0.3X +0.9*90) / (X+90) = 0.7

 

Simplify

 

0.3X + 81 = 0.7X +63

0.4X = 18

thus

 

X = 45