How to reduce the standard deviation to ensure 99% of rods are within tolerance?

[beanryu]

Homework Statement

Plastic rods are cut into nominal length of 6 inches. Actual lengths are normally distributed about a mean of 6 inches and their standard deviation is 0.06 inch.

Question: To what value does the standard deviation need to be reduced if 99% of the rods must be within tolerance?

Homework Equations

sd=standard deviation
u=mean
P(a<X<=b)=F((b-u)/(sd))-F((a-u)/(sd))

The Attempt at a Solution

since they want the possibility of rods to be between u+sd and u-sd to be 0.99, b=u+sd and a=u-sd
and the equation will become
P(a<X<=b)=F((u+sd-u)/(sd))-F((u-sd-u)/(sd))
F(1)-F(-1) doesn't equal to 0.99.

Am I misinterpreting the word tolerance?
I don't know what else to try... please help thank you!

 

[beanryu]

nevermind i misread the problem...