Answer: 
Step-by-step explanation:
For this exercise you need to use the Inverse Trigonometric function arcsine, which is defined as the inverse function of the sine.
Then, to find an angle α, this is:

In this case, you can identify that:

Then, substituting values into
and evaluating, you get that the measure of the angle "C" to the nearest degree, is:

Try using negative numbers
The mountain will be at 80mm long in 50 years because 50 times 1.6mm equals 80