Two monochromatic sources of electromagnetic wave, $P$ and $Q$ emit waves of wavelength $\lambda =20 \mathrm{m}$ and separated by $5 \mathrm{m}$ as shown. $A, B$ and $C$ are three points where interference of these waves is observed. If the phase of a wave generated by $P$ is ahead of wave generated by $Q$ by $\pi /2$ then (given intensity of both waves is $I$)