A monochromatic light source of wavelength x is placed at S. Three slits S1, S2and S3 are equidistant from the source S and the point P on the screen.S1P - S2P = x/6 and S1P - S3P = 2x/3. If I be the intensity at P when only one slit is open. What is the intensity at P when all the three slits are open?