Question

An alternating current has frquency of 50Hz .How many times does it changes it direction in one second?

Answer 1

100 times,as f =1/t cycles/sec. 
hence for 1sec 
f =1 cycles/sec. 
for 50 hz it will be 
50 cycles/sec.hence 100 times changes its direction

Answer 2

100 times ,as f=1/6 cycles/sec
Hence for 1 sec
F=1 cycles /sec
For 50 hz it will be
50 cycles /sec ,hence 100 times
Change its direction