consider the input voltage is sinusoidal of 50Hz,, the duration of current following through 600 ohm resistor, when capacitor discharge from 60 V to 45V , because peak to peak voltage on load resistor is 15V(given).
apply discharging equation of capacitor
45=60exp(-t/RC) (equ. 1)
the off time of diode is 17.6 ms ( calculated by 60sin(314t))
apply time in equation 1 and you will get the value of capacitor is 100uF
