two wave frequencies20Hz & 30Hz travel out from a common point. how will they differ in phase at the end of 0.75 sec?

The phases of the waves in any time ‘t’ is given by 2πft. Where, ‘f’ is the frequency.

So, the phase difference between the waves of 30 Hz and 20 Hz in time 0.75 s is = 2 × π × (30 - 20) × 0.75

= 47.12 radian