whatever% of anything is just (whatever/100) * anything.
thus, what is 80% of M, well is just (80/100) * M,
what is 50% of N? well is just (50/100) * N.
we know they're both equal, thus
[tex]\bf \cfrac{80}{100}M=\cfrac{50}{100}N\implies \cfrac{8M}{10}=\cfrac{5N}{10}\implies \cfrac{4M}{5}=\cfrac{N}{2}
\\\\\\
\cfrac{4\cdot 2}{5}=\cfrac{N}{M}\implies \cfrac{8}{5}=\cfrac{N}{M}[/tex]
