One canned juice drink is 20% orange juice; another is 5% orange juice. how many liters of each should be mixed together in order to get 15 l that is 10% orange juice?

Let's call "x" and "y" the liters that we need:

 x= liters in 20%
 y=liters in 5%
 
  Then, we have the following equations:

 1) x+y=15
 2) 20x/100+5y/100=(10/100)(15) ⇒ 0.2x+0.05y=1.5

 We clear "x" from the equation 1):

 1) x=15-y
 
 Now, we substitute "x" into the equation 2) to find "y":
 
 0.2x+0.05y=1.5
 0.2(15-y)+0.05y=1.5
 3-0.2y+0.05y=1.5
 3-0.15y=1.5
 y=10
 
 Now, we can substitute y=10 into the equation 1) to find the value of "x":

 x+y=15
 x=15-10
 x=5

 How many liters of each should be mixed together in order to get 15 L that is 10% orange juice?
  5 liters of 20% orange juice and 10 liters of 5% orange juice 

The answer is 49/16 or 3 1/16