Page fault rate = 0.3
Hence page hit rate = 0.7
Memory access time = 10 time units
Page fault service time = 125 time units
Let probability of page being dirty = p
Given effective access time = 50 ns
Thus E.M.A.T = 0.7 * [Memory access time] + 0.3*[p * (400 PFST) + (1-p) * PFST]
==> 0.7 * 10 + 0.3 *[400*125*p + 125 - 125p] = 50
==> 7 + 0.3 * [50000p - 125p + 125] = 50
==> 0.3 * [49875p + 125] = 43
==> p = 0.0004 [correct upto 4 decimal places]
NOTE : I have used the expression given in Galvin for effective time calculation..and rest 400 PFST according to the question in case the page is dirty..