(2 4/7-2.5):(1/6-1/21)

По действиям:

1.

$2 \frac{4}{7} -2,5=2 \frac{4}{7} -2 \frac{5}{10}= 2 \frac{4*10}{7*10} -2 \frac{5*7}{10*7}= \\ \\ 2 \frac{40}{70} -2 \frac{35}{70}= \frac{5}{70} = \frac{1}{14}$

2.

$\frac{1}{6} - \frac{1}{21} = \frac{1*21}{6*21} - \frac{1*6}{21*6} =\frac{21}{126} - \frac{6}{126} = \frac{15}{126} =\frac{15:3}{126:3} = \frac{5}{42}$

3.

$\frac{1}{14}:\frac{5}{42}=\frac{1}{14}*\frac{42}{5}=\frac{1}{14:14}*\frac{42:14}{5}= \frac{1}{1} * \frac{3}{5} = \frac{3}{5} =0,6$