SOLUTION
Dev Total Drawings = Rs. 10,000 x 12 = Rs. 1,20,000
Interest on Dev’s Drawings = Rs. 1,20,000 x 12 / 100 x 6 / 12* = Rs. 7,200.
Average Period = Time left after first Drawing + Time left after last Drawing / 2
Average Period = 11.5 + 0.5 / 2 = 6 Months.