a) $$20\frac{3}{4} - 17 = 3\frac{3}{4}$$
б) $$13\frac{8}{11} - 5\frac{2}{11} = (13-5) + (\frac{8}{11} - \frac{2}{11}) = 8 + \frac{6}{11} = 8\frac{6}{11}$$
Ответ: a) $$3\frac{3}{4}$$; б) $$8\frac{6}{11}$$