One canned juice drink is20%
pineapple ; another is 40%
how many liters of each should be mixed together in order to get 30l that is 25% pineapple

So hmmm, first off, let us use the percentage in decimal format, so 20% is just 20/100 or 0.2 and so on

hmm thus 

whatever the amounts of "x" and "y" are, we know they have to add up to 30Litres, thus   x + y = 30

and we also know the concentrated amount of pineapple, have to also add up to 7.5  thus   0.2x + 0.4y = 7.5

thus  

solve for "x", to see how much of the 20% pure juice will be used

what about "y"?  well, y = 30 - x