A large jet uses 7.500 gallons of fuel to fly 1,500 miles what is the unit rate used by the vehicle

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)

The function models the number of accidents

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! )