You copied the answer from textbook which is wrong. It comes to 318 Hz. Use calculator.
The correct answer should be calculated as :
n` = 200 X 340 / 340 - 32
Notice that velocity of echo is twice the velocity of sound i.e whistle in this case. Therefore we negate 2X16 which is 32 from velocity of sound in air. This gives the proper answer 220.77 Hz.