3. If $a, b$ are positive integers, and satisfy $5a+7b=50$, then $ab=$ . $\qquad$
3. 15 .

From the problem, we know
$$
a=\frac{50-7 b}{5}=10-b-\frac{2 b}{5} \text { . }
$$

Therefore, $a=3, b=5$.
Thus, $a b=15$.