Number of complete 90.9 mhz radio waves over a 1.50 km distance

You could answer this right away IF you knew the length of each wave, right ?

Well,  Wavelength = (speed) / (frequency).

Speed = 3 x 10⁸ m/s  (the speed of light)
and
Frequency = 90.9 x 10⁶ Hertz.

So the length of each wave is  3 x 10⁸ / 90.9 x 10⁶  meters.

To answer the question, see how many pieces you have to cut
that 1.5 km into, in order for each piece to be 1 wavelength. 
It'll be

(1,500 meters) divided by (3 x 10⁸ meters/sec) / (90.9 x 10⁶ Hz)

To divide by a fraction, flip the fraction and then multiply:

(1500 meters) times (90.9 x 10⁶ Hz)/(3 x 10⁸ meters/sec)

=   454.5

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