$\left.\begin{array}{l}{\left[\begin{array}{l}\text { Common fractions }\end{array}\right]} \\ {[\text { GCD and LCM. Mutual simplicity }]}\end{array}\right]$

Find all numbers by which the fraction $\frac{5 l+6}{8 l+7}$ can be reduced when $l$ is an integer.
$8(5 l+6)-5(8 l+7)=13$, so the GCD(5l+6, $8 l+7$) is 13 or 1. When $l=4$, we get the fraction $26 / 39$, which is indeed reducible by 13.

## Answer

Only by 13.