One canned juice drink is 25 % orange juice; another is 10 % orange juice. How many liters of each should be mixed together in order to get 15L that is 21 % orange juice?

11 liters of 25% orange juice, 4 liters of 10% orange juice:

1' let x to be 25% orange juice and y be 10% orange juice

2'.25x+.10y = (15).21

3' x+y=15

4' 2.5x+y=31.5 divide all sides by 10

5' - x-y=-15 multiply all sides by - 1

6' 1.5x=16.5

7' x=11

8' y=15-11=4

so 11 and 4 are the answers.