4. 2m-n 5n-2k -- + --= mn nk

4. $$ \frac{2m-n}{mn} + \frac{5n-2k}{nk} = \frac{k(2m-n)+m(5n-2k)}{mnk} = \frac{2mk-nk+5mn-2mk}{mnk} = \frac{5mn-nk}{mnk} $$

Ответ: $$ \frac{5mn-nk}{mnk} $$