Answer :

naǫ
[tex]\frac{1}{3} \times 21=\frac{21}{3}=\frac{7 \times 3}{3}=7 \\ \\ \frac{2}{5} \times 40=\frac{2 \times 40}{5}=\frac{2 \times 8 \times 5}{5}=2 \times 8=16 \\ \\ \frac{4}{7} \times 280=\frac{4 \times 280}{7}=\frac{4 \times 40 \times 7}{7}=4 \times 40=160[/tex]
[tex]A\ fraction\ of\ the\ number\ is\ equal\\to\ the\ product\ of\ the\ fraction\ by\ the\ number.\\\\Therefore:\\\\\frac{1}{3}\ of\ 21=\frac{1}{\not3_1}\cdot\not21^7=1\cdot7=\boxed7\\\\\frac{2}{5}\ of\ 40=\frac{2}{\not5_1}\cdot\not40^8=2\cdot8=\boxed{16}\\\\\frac{4}{7}\ of\ 280=\frac{4}{\not7_1}\cdot\not280^{40}=4\cdot40=\boxed{160}[/tex]