Equations

Mr. Roy buys 100 units of the Unit Trust of India at Rs. 10.30 per unit. He purchases another lot of 200 at Rs. 10.40 per unit. At Rs. 10.50 per unit, he takes up another lot of 400 and a further lot of 300 at Rs. 10.80 per unit. He watches as the price goes down and desires to take up as many units at Rs. 10.25 per unit as would make the average cost of his holding to Rs. 10.50 per unit. Assuming that Mr. Roy always buys units in multiples of 100., find the number of units he purchases at the lowest price of Rs. 10.25 per unit.