The last instalment of your school fees is to be paid. The total fee to be paid was ₹ 24,399 and you have already paid two instalments of ₹ 8,745. So, the estimated (to nearest thousands) amount that you have to pay now is
₹ 24,399 can be rounded off to ₹ 24,000
Similarly, ₹ 8,745 can berounded off to ₹ 9,000
Since the instalment of ₹ 8,745 has been paid twice, total estimated amount paid = ₹ 9000 × 2 = ₹ 18,000
Remaining estimated amount to be paid = ₹ (24,000 - 18,000) = ₹ 6,000