100 students appeared for two different examinations. 60 passed the first , 50 the second and 30 both examinations. Find the probability that a student selected at random failed in both examinations?

• 1/5
  
  Students passed in either 1 or 2 exam=P(AuB)=P(A)+P(B)-P(AnB)=60+50-30=80
  So students failed in both exam=100-80=20
  Hence probability=20/100= 1/5
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