[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]