ANSWER
The smaller angle is

.EXPLANATION
Let the angles be

where y is the smaller angle.
Since they are supplementary, they will add up to 180°.
This implies that,

If one angle is 4 times the other, we can then write the following equation,

We put equation (2) into equation (1) to get,


We divide through by 5 to obtain,

This implies that,

Check the picture below.
now, the "x" is a constant, the rocket is going up, so "y" is changing and so is the angle, but "x" is always just 15 feet from the observer. That matters because the derivative of a constant is zero.
now, those are the values when the rocket is 30 feet up above.

The answer should be
Y=-3x+19