Find an equation for the nth term of the arithmetic sequence. -20, -16, -12, -8, ...

a. an = -20 x 4(n - 1)
b. an = -20 + 4(n - 1)
c. an = -20 + 4(n + 2)
d. an = -20 + 4(n + 1)​

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! )