(2целых2/7-1целая1/3)*21

([latex] \frac{16}{7} [/latex] - [latex] \frac{4}{3} [/latex]) *21 = ([latex] \frac{16*3}{21} [/latex]-[latex] \frac{4*7}{21} [/latex])*21= ([latex] \frac{48-28}{21} [/latex])*21 =  [latex] \frac{20}{21} [/latex]* [latex] \frac{21}{1} [/latex] = 20

(2 2/7-1 1/3)*21=(16/7-4/3)*21=16*3-4*7=48-28=20