If Aston pays $75 each month for 23 months, the total amount he would have paid after 23 months, is:
Since the cost of the motorcycle is $9200, substract $1725 from $9200 to find the amount that is to be paid to cover the balance in full:
Since $7475 are yet to be paid, Aston must pay that amount the last month if he wants to avoid interest charges.
Therefore, the answer is:
Aston should pay $7475 the last month.