Jerry is experimenting with chemicals in the laboratory. he mixes a solution that is 10% acid with a solution that is 30% acid. how much of the 30% acid solution will be needed to make 40 liters of solution that is 25% acid?

10 litres ;)

1/10 x 3/10 = 4/100
5/10 x 5/10 = 25/100

1/10 x 5 = 5/10
(3/10 divided by 3) x5 = 5/10

5y = 5x/3
15y = 5x <-- 20 altogether

40 divided by 20 = 2

2x5 = 10litres
2x15 = 30litres

(Sorry it's written really weirdly, I can't type anything on my phone x'D )