Two waves, each having a frequency of $100 \mathrm{Hz}$ and a wavelength of $2·0 \mathrm{cm},$ are travelling in the same direction on a string.  the 

(a) at the same place, if the second wave was produced $0·015 \mathrm{s}$  after the first one then phase difference between the waves is $\alpha \pi \mathrm{rad}$ .

(b) If the two waves were produced at the same instant but the first one was produced with a distance $4·0 \mathrm{cm}$ behind the second one then phase difference between the waves is $\beta \pi \mathrm{rad}$ .

(c) If each of the waves has an amplitude of $2·0 \mathrm{mm},$ then the amplitudes of the resultant waves in part (a) and (b) are $\gamma \mathrm{mm} \mathrm{and} \mathrm{\sigma } \mathrm{mm}$.

Find $\alpha +\beta +\gamma +\sigma$.