Exact or approximate?

1 lakh people visited the book fair. Headlines like this don't mean exactly 1,00,000. Often an approximate number is enough.

In the 2011 census Chintamani had 76,068 people. Saying about 75,000 gives the idea and is easier to remember.

But not always. Bappi is asked to call and check whether the Toofan Express is on time. He rounds off the phone number, because my teacher said efficient people use rounded-off numbers! A phone number has to be exact. So do money in a bank, a medicine dose and an emergency number.

Rounding up means choosing a number more than the real one. A school has 732 people, so the principal orders 750 sweets, not 700; with 700 someone would get none. You round up when running short would be a problem: food for guests, cloth for the tailor.

Rounding down means choosing a number less than the real one. An item costs ₹470, and the shopkeeper says around ₹450, not around ₹500. You round down when saying too much would be the problem.

So whether to round up or down is not a rule of arithmetic. It depends on what the number is for. Think of your own examples: when should you round up, when down, when does either do, and when must it be exact?