At A, PA-QA =PQ = 5m\\\\(=\\dfrac{20}{4}=\\dfrac{\\lambda }{4}=\\Delta x\\\\)\\\\(\\Delta \\phi '=\\dfrac{2\\pi}{\\lambda }\\Delta x =\\dfrac{2\\pi}{\\lambda }\\cdot \\dfrac{\\lambda }{4}=\\dfrac{\\pi}{2}\\\\)\\\\(\\because \\\\) Phase of Q is greater by \\\\(\\dfrac{\\pi}{2}\\\\) than P\\\\(\ herefore \\\\) Net phase difference \\\\(\\phi =\\dfrac{\\pi}{2}-\\dfrac{\\pi}{2}=0\\\\)\\\\(I_A=4I'\\\\)At \\\\(B, \\phi =\\dfrac{\\pi}{2}, I_B=2I'\\\\)At \\\\( C \\,\\,\\,\\Delta x=QA-PA=\\dfrac{\\lambda }{4}\\\\)\\\\(\\phi '=\\dfrac{\\pi}{2}\\\\)Net phase difference \\\\(\\phi =\\dfrac{\\pi}{2}+\\dfrac{\\pi}{2}=\\pi\\\\)\\\\(I_c=0\\\\)\\\\(I_A:I_B:I_C=4I':2I':0\\\\)\\\\(=2: 1:0\\\\)