projectile problem?

Question

A projectile's launch speed is five times its speed at maximum height. Find launch angle.
How to get the answer of 78.5 degrees?

Thanks

Answer 1

(launch speed) * (cosinus launch angle) = (speed at maximum height)

angle = arccos (0.2) = 1.3694 = 78.46 degree

Note:

The question and the answer doesn't depend on gravity. It would be the same in the Moon or some another planet.

Answer 2

Acceleration due to gravity is 9.8 meters per second squared, or 32 feet per second per second.
In this case, it's negative/deceleration.
Sorry, I don't have a complete answer.
You need a trigonometry table.