$40 per month
You could do this two ways:
Divide 280 by the 8 months to get the original charge per month including the discount, which would be $35 per month. He would've been charged an additional $5 for every month, so add 5 to the 35 to get $40 per month.
He would've been charged an additional $5 for every month, so multiply 5 by 8 to get 40, then add that to 280 to get 320. That's how much he would've paid without the discount over 8 months. Divide that by the 8 months, and you get $40 per month.
