Ravi wants to add 48 and 37 quickly. He rounds 48 up to 50 and 37 up to 40, adds those to get 90…

Why

Ravi's method is genuinely useful — rounding to the nearest ten is exactly how you'd quickly check whether an answer feels about right, and 90 is a perfectly good estimate. The trouble is he stopped at the estimate and called it the exact answer, without ever adding the real numbers. Work it out directly: 48 + 37 = 85. His rounded estimate of 90 is off by 5, because he rounded BOTH numbers up (48 to 50 added 2, 37 to 40 added 3), so the errors stacked instead of cancelling. If you round a shopping bill of ₹48 and ₹37 to ₹50 and ₹40 in your head, you'd expect to pay about ₹90 — but the shopkeeper's bill would actually read ₹85, and handing over ₹90 would be paying 5 rupees too much. Sunita is right: rounding tells you the ballpark fast, but only adding the real numbers tells you the exact one.