21) $$ \frac{2}{5} $$ + $$ \frac{1}{4} $$ - 0,4 = $$ \frac{8 + 5}{20} $$ - $$ \frac{4}{10} $$ = $$ \frac{13}{20} $$ - $$ \frac{2}{5} $$ = $$ \frac{13 - 2 \cdot 4}{20} $$ = $$ \frac{13 - 8}{20} $$ = $$ \frac{5}{20} $$ = $$ \frac{1}{4} $$
Ответ: $$ \frac{1}{4} $$