backwards Normal distribution problem

I feel like I should be able to do this but I feel absolutely lost.

**Assume that the weight, X, of male college students is normally distributed. Assume further that P(X <= 160) = 1/2 and P(X <= 140) = 1/4.**

a.) What find the value of 'mu' and 'sigma'

b.) Calculate P(X >= 190)

c.) Calculate what percentage of students who weigh over 190 pounds will weigh more than 200 pounds.

I think b and c should be simple enough once I have mu and sigma but how do I get those? Any help would be appreciated. (Bow)