( [latex] \frac{1}{24} [/latex] + [latex] \frac{1}{42} [/latex] ) · 21

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

[latex](\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{1\frac{3}{8}}[/latex]