calculate the 95% confidence interval for u given X=100, s=25, and N=101
CI95 = mean + or - 1.96(sd/√n)
...where + or - 1.96 represents the 95% confidence interval using a z-table, sd = standard deviation, √ = square root, and n = sample size.
Substitute the values you know into the formula and go from there.
I hope this will help get you started.