net effect on profits due to the change in the method of depreciation.

W ltd. leased in January 20X1 for ten years a mine containing an estimated quantity of 500000 tonnes of minerals. The amount paid was $1500000. The company decided to provide depreciation on the Straight line basis. The output was as under:
20X1- 30000 tonnes, 20X2- 50000 Tonnes, 20X3- 40000 tonnes, 20X4- 60000 tonnes, 20X5- 65000 tonnes.
In 20X5, the company decided to change over to the Depletion method and to give effect to it retrospectively from the very beginning. Calculate the net effect on profits due to the change in the method of depreciation.