[tex]\bf \begin{array}{lccclll}
&amount(gallons)&juice&\textit{juice amount}\\
&--------&-----&-----\\
\textit{50\% punch}&4&0.50&(4)(0.50)\\
\textit{pure juice}&x&1.00&(x)(1.00)\\
-----&-----&-----&-----\\
mixture&4+x&0.60&(4+x)(0.60)
\end{array}[/tex]
notice, that, pure juice is 100% juice, dohhh, thus 100/100 = 1.00
50% is 50/100 or 0.50 in decimal format
so..... whatever those two quantities amount to, that is, the 50% and pure juice, or (4)(0.50) + (x)(1.00)
they will equal the mixture desired 60% juice, or 0.60, namely (4+x)(0.60)
thus (4)(0.50) + (x)(1.00) = (4+x)(0.60)
solve for "x"