Three waves of equal frequency having amplitudes $10\mu m,4\mu mand7\mu m$  arrive at a given point with successive phase difference of $\frac{\pi }{2}$ . The amplitude of the resulting wave in $\mu m$  is given by ..
 

The wave 1 and 3 reach out of phase. Hence resultant phase difference between them is $\mathrm{\pi }$  .
Resultant amplitude of 1 and 3  $=10-7=3\mu m$
This wave has phase difference of $\frac{\pi }{2}$  with  $4\mu m$
Resultant amplitude $=\sqrt{3^2+4^2}=5\mu m$