Question

how much of a 25% solution should to be added to 35% solution to get 40 liters of 28% solution

Answer 1

Let the ratio of 35% and 25% solutions be x:(1-x)
To get a 28% solution,
35*x + 25*(1-x) = 28
35x-25x = 28 - 25
x = 3/10 (35%)
(1-x) = 1-3/10 = 7/10 (25%)
35% solution required for 40 litres of mixture
= 40*x = 12 litres
25% solution required for 40 l. of mixture
= 40(1-x) = 40 * 7/10 = 28 litres.
Check: 28+12=40 litres.

Answer 2

or

.25x + .35(40-x) = .28(40)
multiply by 100 and solve
x = 28