1. The problem statement, all variables and given/known data The masses of suspended particles in a sample of water taken from a lake can be assumed to be a random variable which is normally distributed with mean 2.17 and variance 0.979. Find the probability that out of 4 samples of lake water known to contain less than 1.8 mg of suspended particles , at least 3 samples contain less than 1.6 mg of suspended particles. 2. Relevant equations 3. The attempt at a solution probability of a sample less than 1.6 mg, P(X<1.6)=0.28227 X-B(4, 0.28227) P(X>=3)=P(X=3)+P(X=4)=0.071 Probability of all 4 samples contain less than 1.8 mg of suspended particles = (0.3542)^4 = 0.015 So the required probability = 0.071/0.015 = 4.7 obviously that's wrong. Would appreciate if someone can point me to my errors.