50% of all the cakes jenny baked that week were party cakes, 1 fifth were fruit cakes, and the remainder were sponge cakes. what percentage of cakes were sponge cakes?

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )