well, we know that he makes $60 weekly, that'd be the 100%, we also know that he saves 30% of that and the rest, well, 100% - 30% = 70%, so 70% he spends it on food.
if we take 60 to be the 100%, what is 70% of it?
[tex]\begin{array}{ccll} amount&\%\\ \cline{1-2} 60 & 100\\ x& 70 \end{array} \implies \cfrac{60}{x}=\cfrac{100}{70}\implies \cfrac{60}{x}=\cfrac{10}{7} \\\\\\ 420=10x\implies \cfrac{420}{10}=x\implies 42=x[/tex]
