Can anybody tell me why the answer is 21%?

d) Revaluation reserve. 4. The management of Topnotch Ltd wish to compare the effects of their "operating" decisions, on the profitability of its assets, against their major competitor; Hightop Ltd. From the information provided below calculate the appropriate return on asset, they should use. (Round to the nearest %) Net profit after-tax $109 000 Net profit before interest and tax $152 000 Beginning balance of total assets $620 000 Ending balance of total assets $840 000 a) 15% b ) 21% C 18% d) 13%