Zara rounds 7,654,321 to the nearest 100,000 and writes 7,600,000. What mistake did she make?
A decimal number has place-value columns after the point too: tenths, then hundredths, then thousandths. To round a decimal, first decide which place you want to keep. Then look at the very next digit to its right.
Round 3.4567 to 2 decimal places. Keep the tenths and hundredths digits: 3.45. Look at the next digit, which is 6. Since 6 is 5 or more, the hundredths digit rounds up by one, giving 3.46.
The same rule works for big whole numbers. To round 7,654,321 to the nearest 100,000, keep the digit in the hundred-thousands place (6) and look at the digit right after it (5). Since 5 is 5 or more, that digit rounds up, giving 7,700,000.
Rounding is also useful for quick estimates. To estimate 4.83 × 2.17, round each number first: 5 × 2 = 10. That is close enough to check your exact answer makes sense.
Rounding any number means keeping the place you want and checking only the very next digit to decide whether it goes up.