How many liters of 30% alcohol solution must be mixed with 40 liters of a 70% solution to get a 40% solution?

We have 40 liters of 70% alcohol
How much 30% must we add to make 40% alcohol?
.70 * 40 + .30x = .40 * (40 + x)
28 + .30x = 16 + .40x

12 = .10 x
x = 120 liters of 30% must be added

Double-Check
40 ltrs .70 alcohol = 28 liters alcohol 12 water
120 ltrs. .30 alcohol = 36 ltrs. alcohol 84 water

adding both we get
64 ltrs. alcohol 96 ltrs. water 
160 liters total
64 / 160 = .40 or 40% alcohol solution 
correct!!!
Source:1728.com/mixture.htm

Did you put in a negative answer? no matter if you + - ÷ or ×, it will be negative example: -21 ÷ 3 = -7