Paper 6 May/June 2011 MATHEMATICS (9709/61) A Levels
لطفا برای اطمینان از عملکرد و تجربه بهتر از مرورگرهای مدرن و به روز مانند کروم یا فایرفاکس استفاده کنید.

The lengths, in centimetres, of drinking straws produced in a factory have a normal distribution with mean $\mu $ and variance 0.64. It is given that 10% of the straws are shorter than $20{\text{ }}cm$.
a) Find the value of $\mu $.
b) Find the probability that, of 4 straws chosen at random, fewer than 2 will have a length between $21.5{\text{ }}cm$ and $22.5{\text{ }}cm$.