13. $\lg \sqrt[3]{25}$, if $\lg 64=a$.

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13. $\lg \sqrt[3]{25}$, if $\lg 64=a$.
13. $\lg 64=6 \lg 2 ; \lg 2=\frac{a}{6} ; \lg 5=\lg \frac{10}{2}=1-\lg 2 ; \lg \sqrt[3]{25}=\frac{6-a}{9}$.