A loan is being repaid by 15 annual installments of $1000 each. Interest is at an effective rate of 5%. Immediately after the 5^th installment is paid, the loan is renegotiated. The revised schedule calls for a 6^th installment of $800, a 7^th installment of $(800 + K), with each subsequent installment increasing by K over the previous payment. The period of the loan is not changed. Determine the amount of the last installment