Ok so 1/f is the period which is 14.29ms. Then 30 is 8.333% of 360 so 8.333% of 14.29ms is 1.19ms and thats how long it conducts for right? So it supplies current to the load for 13.1ms?Perhaps ask yourself some questions such as :-
Can I explain how this circuit works? What is the function of each component? What is happening at different stages of the supply AC cycle?
If the frequency is 70 Hz what is the period (in ms) of the AC cycle?
If the AC cycle is also equivalent to 360° how many ms are equivalent to 30°?
If the diode conducts for 30° of the AC cycle then for how many degrees does the capacitor supply current to the load?
If I know the no. of degrees during which the capacitor supplies load current what is that equivalent to in ms?