Hello!
if he bought it with no interest applied, then the last payment will be $1400
1670 - 100 = 1570
if he has to pay it off in 18 months (1 and a half years), and only pays $10 per month, that means he will pay off $170 before the last payment is due.
1570 - 170 = 1400
so his last payment will have to be $1400 to pay it off completely
I hope this helps, and have a nice day!