May 2006 Question 2 The head circumference of 3-year-old boys is known to be Normally distributed with mean 49.7cm and standard deviation 1.6cm. Find the probability of having a head (A) over 51.5cm (B) between 48cm and 51.5cm 4 3-year-old boys are selected. Find the probability that exactly one of them has a head size between 48cm and 51.5cm
May 2006 Question 2 Solution X~N(49.7,1.6 2 ) (i)(A) P(X>51.5)=P(Z>51.5)=P(Z> ) P(Z>1.125)=1-P(Z<1.125) =1-Φ(1.125) = =0.1304
(i)(B) P(48≤X≤51.5)=P(X≤51.5)-P(X≤48) =P(Z≤1.125)-P(Z≤ ) = Φ(1.125)- Φ(-1.063) = (1- Φ(1.063)) = ( ) = =0.7257
(ii) P(48≤X≤51.5)= n=4 Therefore X~Bi(4,0.7257) P(X=1)= =0.0599
Key Points Look carefully at the question. See if you can use anything you’ve calculated before. Just because the binomial distribution is not on the S2 syllabus directly, doesn’t mean they won’t ask you to use it.