the sample size needed

Question

A quality engineer is interested in estimating the mean time required to assemble a bat code scanner. If the engineer wishes to be 95% confident that the error is estimating the mean time is less than 0.25 minutes, and she knows from the past experience that the standard deviation of the assembly time is 0.75 minutes; the sample size she would need is?

Answer 1

Our margin of error formula is: z*(standard deviation/√n), at a 95% confidence interval z=1.96

.25<1.96*(.75/√n)

get n by itself:

.17006<1/√n

√n<1/.17006

√n<5.88

n<34.57

n=35