Question

One canned juice drink is20%

Answer 1

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 $\bf \begin{array}{lccclll} &amount&concentration& \begin{array}{llll} concentrated\\ amount \end{array}\\ &-----&-------&-----\\ \textit{20\% pineapple}&x&0.2&0.2x\\ \textit{40\% pineapple}&y&0.4y&0.4y\\ -----&-----&-------&-------\\ mixture&30&0.25&7.5 \end{array}$

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  $\bf \begin{cases} x+y=30\implies \boxed{y}=30-x\\ 0.2x+0.4y=7.5\\ ----------\\ 0.2x+0.4\left( \boxed{30-x} \right)=7.5 \end{cases}$

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

what about "y"?  well, y = 30 - x