Underlined = the place we round toHighlighted = the digit we look at
Roll the ball down the hill to the nearest landmark. Learn why 5 rounds up, then use rounding to estimate answers fast.
Start here. Rounding swaps a number for a nearby, friendlier number. Think of a hill between two landmarks, like 40 and 50. A ball on the hill rolls to whichever landmark is closer. 47 rolls to 50. 43 rolls to 40.
The digit rule. To round to the nearest 10, look at the ones digit. If it is 0, 1, 2, 3 or 4, round down. If it is 5, 6, 7, 8 or 9, round up. For the nearest 100, look at the tens digit. For the nearest 1,000, look at the hundreds digit. For a whole number, look at the tenths digit.
Why does 5 round up? A number like 35 is exactly halfway between 30 and 40, so it is not closer to either one. People agree to send halfway numbers up, so 35 rounds to 40. This game always uses that rule.
Estimating. Round first, then do the sum in your head. To estimate 38 + 47, round to 40 + 50 = 90. The real answer is 85, which is close to 90. For 38 × 6, round 38 to 40, then 40 × 6 = 240. An estimate does not need to be exact. It tells you if an answer makes sense.
Try this above. In Play, drag the ball to the very top of the hill. Which way does it go? Switch to the nearest 100 and try 350 and 349. Then play Missions and use the Hint button to see the hill.
Find the multiple of 10 that is closest to your number. 62 is closest to 60. 68 is closest to 70.
Numbers ending in 5 are exactly halfway, so we need a rule. The usual rule is to round up. So 15 becomes 20 and 250 becomes 300 when rounding to the nearest 100.
Look at the tenths digit, the first number after the point. 4.6 has a 6, so it rounds up to 5. 4.3 has a 3, so it rounds down to 4. 4.5 is halfway, so it rounds up to 5.
An answer is reasonable when it is close to your estimate. If you estimate 40 + 50 = 90 and someone says the answer is 190, that cannot be right.
Just starting uses the nearest 10. I know the basics adds the nearest 100 and estimating sums and differences. Challenge me adds 1,000, 10,000, decimals and products.
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