On one day a furniture dealer sold a sofa at a loss of 12% and an armchair that had cost him ₹9000 at a profit of 25%. Taken together the two sales left him ₹750 better off. The sofa had cost him (in ₹):

The armchair brought in a profit of $9000\times 0.25$ = ₹2250. Since only ₹750 survived, the sofa must have lost ₹1500, and ₹1500 is 12% of ₹12500. (Check: $12500\times 0.88$ = ₹11000 and $9000\times 1.25$ = ₹11250, so ₹21500 of stock fetched ₹22250.) Dividing the whole ₹2250 by 0.12 gives ₹18750 and forgets to leave ₹750 behind; dividing the ₹750 by 0.12 gives ₹6250 and treats the net gain as the sofa's loss; ₹21500 is the two cost prices added together.