Cell phone Plan A costs $70 per month and comes with a $100 phone. Cell phone Plan B costs $50 per month and comes with a $200 Phone. How many months is it until the cost for both plans is the same?

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! )