A tablet marked at ₹18000 was finally billed at ₹12240 after two successive discounts, the second of them being 15%. The first discount was what percent?

Undo the second discount first: ₹12240 is 85% of the price after the first discount, so that price was 12240/0.85 = ₹14400. Then 14400/18000 = 0.80, a first discount of 20%. The overall reduction is 5760/18000 = 32%, but discounts do not add, so 32 − 15 = 17 is wrong — the 15% was taken on ₹14400, not on ₹18000.