Hello cruxkitty

Suppose that at time minutes, the angle of elevation of the balloon from the range finder is , and that the height of the balloon is feet.

Then (1)

Now the rate at which the balloon is rising is feet per minute.

So, find an expression for by differentiating (1) with respect to . (You'll need to differentiate the RHS with respect to first and then multiply the result by .)

Now represents the rate at which is increasing in radians per minute. And you know the value of this when , don't you? So ...?

Can you see what to do now?

Grandad