Finding standard deviation

Question

You measure 49 children's birth weights. It is a normal distribution curve. It has a mean of 3.1 feet and .18 chance of getting 4.2 feet. What is the standards deviation?

Answer 1

The BEGINNING of the problem is FLAWED!!!!

Weight is measured in pounds not feet.

Height is measure in feet. There is no way

they can be that tall at birth!!!

Anyway....

the z-score per the normal table is for which Prob(z<k) = 0.18 is k=-0.91537, which means

the stat must be less than the mean. But here, the stat is greater than the mean, which

makes the standard deviation negative. No can do...

so to force the statistic to be positive, we add 1/2 to the probability.

then we need Prob(z< k) = 0.68

THe z-score is now k=0.467699.

0.467699 = 1.1/S

S = 1.1/0.467699 = 2.35194