3. There are two roads $O M, O N$ intersecting at a $30^{\circ}$ angle. Along the direction of road $O M$, 80 meters from $A$ is a primary school. When a tractor travels along the direction of $O N$, areas within 50 meters on both sides of the road will be affected by noise. Given that the speed of the tractor is 18 kilometers/hour. Then, the time during which the tractor traveling along $O N$ will cause noise impact to the primary school is $\qquad$ seconds.
3. 12 .

As shown in the figure, when the tractor reaches point $B$, it starts to have an impact, and when it reaches point $C$, the impact ceases. Thus, $A B = A C = 50$ meters. Draw $A D \perp$ $O N$ from $A$. Given that $\angle A O D = 30^{\circ}$, we know that $A D = \frac{1}{2} O A = 40$ meters. Therefore, $B D = \sqrt{A B^{2} - A D^{2}} = 30$ meters, and thus $B C = 60$ meters. Since the speed of the tractor is 18 kilometers/hour $= 5$ meters/second, the duration of the noise impact is $t = \frac{60}{5} = 12$ seconds.