A college has raised 75% of the amount it needs for a new building by receiving an average donation of Rs. 600 from all the faculties. If faculties are 60% of the total staff, then find the average donation should college get from the remaining staff so that college could raise the exact amount for the new building ?

Let total amount needed be T. Faculties (60% of staff) contributed 75% of T at average Rs. 600. Let total staff be S. Then 0.6S × 600 = 0.75T, which gives 360S = 0.75T, or T = 480S. Remaining 40% staff must contribute 25% of T, which is 0.25 × 480S = 120S. Their average = 120S ÷ 0.4S = Rs. 300. Use weighted averages carefully.