A showroom sells a phone for ₹24000 in cash, or on a 'pay-in-instalments' plan of ₹2200 a month for 12 months with nothing paid upfront. Reckoned on the cash price, the extra amount the instalment buyer pays is what percent?

The instalment buyer pays $12\times 2200$ = ₹26400, which is ₹2400 more than the cash price. As a percent of the cash price that is 2400/24000 = 10%. (Measuring the ₹2400 against the ₹26400 actually handed over gives 9.09% — the comparison must be with the cash price, which is what the extra is being charged on. The true annual rate is higher still, because most of the money is paid before the year is out.)