Question

assume point of the fire to be origin and the bullet moves in

Answer 1

lets assume R = 4 , since the direction is already provided and won't change with range.

Now, when R = 4, position of bullet is (3,1)
Now imagine an arc section with the horizontal x axis, the direction of bullet and y axis as the height or arc length.

here, radius = 3, arc sector = 1
angle = 1/3

$\frac{1}{3} \times \: \frac{\pi}{180} = \alpha \times \pi \div 180$
$\alpha = \frac{1}{3}$
think so