A kite is flying at a height of 40m. A boy is flying it so that it is moving horizontally at a rate 3m/sec. If the string is taut, at what rate is the string being paid out when the length of the string released is 50m?

Question

ybarrap · Answer

I'm not certain on the approach, but this is what the situation looks like 
|dw:1426389948134:dw|
The horizontal change in length of the string ( "c" in picture) is 3 m/sec. The vertical change is 10 m.

What else do we know?