Question

( $frac{1}{24}$ + $frac{1}{42}$ ) · 21

Answer 1

=(7/168+4/168)*21=11/168*21=231/168=1 63/168=1 3/8
вот и все

Answer 2

$(frac{1}{24} + frac{1}{42}) cdot 21 = frac{1}{24} cdot 21 + frac{1}{42} cdot 21 = frac{21}{24} + frac{21}{42} = frac{3 cdot 7}{3 cdot 8} + frac{21}{2 cdot 21} =\\= frac{7}{8} + frac{1}{2} = frac{7}{8} + frac{1 cdot 4}{2 cdot 4} = frac{7}{8} + frac{4}{8} = frac{7+4}{8} = frac{11}{8} = boxed{1frac{3}{8}}$