In an arithmetic sequence, the first term, ai, is equal to 9, and the fourth term, 84, is equal to 24. Which number represents the common difference of the arithmetic
sequence?
d=3
d=4
d=5
d=6

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! )