Kavita spends 30% of her monthly take-home pay on rent, 22% on groceries and 15% on her son's school fee, and saves the rest. If she saves ₹9,900 a month, what is her monthly take-home pay?

Her three outgoings take 30 + 22 + 15 = 67% of the pay, so the savings are 100 - 67 = 33%. Hence 0.33 $x$ pay = 9900, giving pay = 9900/0.33 = ₹30,000. Check: 9000 + 6600 + 4500 = 20100, and 30000 - 20100 = 9900. Each wrong option divides 9,900 by a named outgoing instead of by the leftover share - ₹33,000 by 30%, ₹45,000 by 22%, ₹66,000 by 15%.