It's 50 feet.

Using Pythagoras, it's the square root of 30*2 + 40*2. It effectively a 3, 4, 5 right angle triangle.

It's not worded super clearly, but they mean that the rope is being tied diagonally from the top of the building to a point 30 feet away from the building's base (horizontally). Like you, I initially interpreted the question to mean that the building was 30 feet above ground level, which left me a bit confused.

No.

A2 + b2 = c2

And this is pre-calculator so the number is one of the squares that the kids would have memorized. 4^2 + 3^2 = 25 = 50 feet.