Answers

2016-03-09T17:31:51+05:30
Frequency =
No. of oscillations/time taken
= 20/4
= 5 Hz.

Or

Frequency = 1/Time period
Time period = 4/20 = 0.2
Frequency = 1/0.2 = 5 Hz
2 4 2
2016-03-09T17:33:27+05:30
frequency = 1/time period 
so,  time period = 4/20 = 0.2 sec
therefore,
frequency = 1/t = 1/0.2 = 5 Hz

i hope it helps.................. 
4 4 4
mei ulta likh rhi thi !
lol ! :)
Yep! now that's perfect ! ;)
perfect :-)
hmm :)