On April 1, 2021, John Vaughn purchased appliances from the Acme Appliance Company for $1,200. In order to increase sales, Acme allows customers to pay in installments and will defer any payments for six months. John will make 18 equal monthly payments, beginning October 1, 2021. The annual interest rate implicit in this agreement is 24%. Required: Calculate the monthly payment necessary for John to pay for his purchases.

Solution in the text book is

PV = ? x 0.90573* = 1,200 --> PV = $1,200 / 0.90573* = $1,325

(*) Present value of $1: n=5, i = 2% (from Table 2)

My question is why n = 5, not 6. Can you explain it?