in the United States of America the average newborn baby wei

in the United States of America, the average newborn baby weighs 7.5 pounds. The standard deviation on these weights is 1.2 pounds. In what weight range, rounded to the nearest tenth of a pound, do 90% of the babies born fall?

Solution

P ( Z < x ) = 0.9
Value of z to the cumulative probability of 0.9 from normal table is 1.282
P( x-u/s.d < x - 7.5/1.2 ) = 0.9
That is, ( x - 7.5/1.2 ) = 1.28
--> x = 1.28 * 1.2 + 7.5 = 9.0384 ~ 9.04 pounds