The top of one tree is 16 feet higher than the top of another tree. the heights of the two trees are in the ratio 3: 4. how tall is the tallest tree?

Let one of the trees be  x.
Other        is              = (x + 16)

Ratio :      x :  (x + 16)  =  3:4

Therefore:            x / (x + 16)  =  3/4      Cross Multiply

4*x  =  3*(x + 16)
4x  =    3x + 48
4x - 3x  = 48
x = 48.

The taller tree is (x + 16)  =  (48 + 16) = 64

Taller tree is 64 feet.