I've done these excercises before, but It's not really fresh in my memory anymore..

Suppose $\displaystyle X_1,\cdots, X_n$ independant normal distributed stochasts with $\displaystyle \mu=5,\sigma^2=10$

How do we find c such that $\displaystyle P(5-c\leq \overline{X}\leq 5+c)=0.6$