Q:A random sample of size 15 from a population which is n( μ, σ2 )

yields

=3.2 and S^2=4.24 Find 90% confidence interval for σ2.

I know confidence interval for σ2 is [ns^2/b, ns^2/a]

but how values to determine a and b??? in my book they have assign a=6.57 and b=23.7

BUT HOW??? please tell .. i have to try another question of same type ...

thanks .........!!