Hi, I was trying to solve this question but my answer is different to the one given by my textbook. "The weight of a randomly chosen plastic washer is normally distributed with mean 5g. Calculate the standard deviation in grams given that the probability that a randomly chosen washer weighs less than 3g is 0.123." I said X is equivalent to a normal distribution with mean 5 and variance A. As X is a continuous distribution, the binomial probability of it being less than 3g can be approximated to the normal distribution being less than 2.5g, so: P(X<2.5) = 0.123 P(z<(2.5-5)/sqrtA) let sqrtA = B = standard deviation P(z<-2.5/B) 1 - P(z<2.5/b) = 0.123 P(z<2.5/B) = 0.877 Referring to statistical tables for the normal distribution I found 2.5/B to equal 1.16 This yields a value of B (standard deviation) to be 2.155 But my answer book says 1.72. Can anyone see where I'm going wrong?