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3a = 5b => a = \frac{5b}{3} ;\\
\frac{3a}{10b}=\frac{3\cdot \frac{5b}{3}}{10b}=\frac{3\cdot5b}{3\cdot10b}=\frac{1}{2}." alt="\frac{a}{b}= \frac{5}{3} = > 3a = 5b => a = \frac{5b}{3} ;\\
\frac{3a}{10b}=\frac{3\cdot \frac{5b}{3}}{10b}=\frac{3\cdot5b}{3\cdot10b}=\frac{1}{2}." align="absmiddle" class="latex-formula">
Ответ: а) 1:2.