The frequency of a wave is just the reciprocal of the period.
For this one . . .
Frequency = 1/(period) = 1/(5 sec) = 0.2 per sec (0.2 Hz) .
Answer:
the answer is B: earth takes to rotate once on its axis
You don't need to worry about the 10 year bit with this question. Just grab a calculator and divide 100/2, then the answer to that (50) by 2 etc and keep dividing by 2 until you get down to 6.25.
The answer ends up being 4 half lives :)
If you don't understand what a half life is please let me know :)
