An air-cooler is sold at a discount of 10% on its marked price, and GST of 18% is then charged on the discounted price. The customer pays ₹15930. The marked price (in ₹) is:

Strip the tax first: 15930/1.18 = ₹13500 is the discounted price, and that is 90% of the marked price, so the marked price is 13500/0.9 = ₹15000. Stopping at ₹13500 answers the wrong question, and netting the two rates to +8% (15930/1.08 = ₹14750) is wrong because the 18% is charged on the reduced price, not on the marked price — charging it on the marked price would give ₹17700.