An ac voltage is represented by $E=220\sqrt{2}cos\left(50\pi \right)t$. The current becomes zero times in 1 sec.

20 25 50 60

50

$E=E_{0}cos\omega t$ $\therefore \omega =50\pi$ $2\pi f=50\pi \Rightarrow f=25Hz$ In one cycle ac current becomes zero twice. Therefore, 50 times the current becomes zero in 1 s