Here is the text of my problem:

A boy is flying a kite at a height of 150ft. If the kite moves gorizontally away from the boy at 20ft/s, how fast is the string being paid out when the kite is 250ft from him?

Given:

- y=150ft
- x=250ft
Find:

Work:

- by the theorem of pythagoras:

The book goves an answer of

16ft/s. Can someone give me a hint of where I might have gone astray? Thanks!