A3. If $a$ and $b$ are positive integers such that $a=0.6 b$ and $\operatorname{gcd}(a, b)=7$, find $a+b$.
Solution: $\frac{a}{b}=\frac{6}{10}=\frac{3}{5}=\frac{3 \times 7}{5 \times 7}=\frac{21}{35}$. Thus, $a=21$ and $b=35$, so $a+b=21+35=56$.
Answer: 56 . 56.