Weight of a species of bird is normally distributed with a s

Weight of a species of bird is normally distributed with a standard deviation of 2 grams. If 30% of birds weigh less than 10 grams, what ist he population mean weight of these birds? Give your answer as a number without units

Solution

P(X<10) = 0.3

--> P((X-mean)/s <(10-mu)/2) = 0.3

--> P(Z<(10-mu)/2) = 0.3

--> (10-mu)/2 = -0.52 (from standard normal table)

So 10-mu = -0.52*2=-1.04

So mu = 10+1.04 =11.04

Answer: 11.04