String a is 30cm longer than string b. string b is 60cm longer than than string c. the total length of all 3 strings is 3m. find the length of string c?

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! )