One canned juice drink is 25 percent orange juice another is 10 percent orange juice how many literally of each should be mixed together in order to get 15l that is 21 percent orange juice ?

X - 25% juice
y - 20% juice

We need 15 liters of 21% juice
A) x + y = 15
B) .25x + .20y = 15 * .21
Multiplying A) by -.2
A) -.2x -.2y = -3.0 Then adding to B
B) .25x + .20y = 3.15
.05x = .15
x = 3 liters of 25% juice
y =12 liters of 20% juice

Source:
http://1728.org/mixture.htm