How much of a 70% orange juice drink must be mixed with 5 gallons of a 10% orange juice drink to obtain a mixture that is 50% orange juice?

We have 5 gallons of 10% juice
We'll add "x" gallons of 70% to make 50% juice.

.10 * 5 + .70x = .50 * (5 + x)
.5 + .70x = 2.5 + .5x
.20x = 2
x = 10 gallons of 10%juice

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