Rounding and Estimation

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Concept Review

Rounding to the Nearest Ten: Finding the Closest Neighbor

Imagine you're standing on a number line between two lamp posts. One lamp post is at 20, and the next one is at 30. You're at 24. Which lamp post are you closer to? This is exactly what rounding to the nearest ten helps us figure out!

Rounding is like finding the "closest neighbor" for any number. When we round to the nearest ten, we're asking: "What multiple of 10 is this number closest to?"

The Rounding Rules

Here's how to find the closest ten for any number:

•Look at the ones digit (the last digit)
•If it's 0, 1, 2, 3, or 4 → Round down
•If it's 5, 6, 7, 8, or 9 → Round up

Let's try this with the number 67. The ones digit is 7. Since 7 is greater than 5, we round up to the next ten, which is 70. We can check: 67 is only 3 steps away from 70, but 7 steps away from 60. So 70 is definitely the closer neighbor!

🔑 Key Insight

The number 25 is exactly halfway between 20 and 30. But we always round 25 up to 30, not down to 20. Why? Mathematicians agreed on this rule so everyone rounds the same way. When in doubt, round up!

Real-World Rounding

Think about a classroom with 43 students going on a field trip. The bus company needs to know roughly how many kids are coming. Instead of saying "43 students," the teacher might say "about 40 students" – that's rounding 43 down to the nearest ten. It gives a quick, useful estimate without getting caught up in exact details.

Round Down

32 → 30

18 → 20

54 → 50

Round Up

37 → 40

19 → 20

85 → 90

Key Takeaway

Just like finding the closest lamp post when you're walking at night, rounding helps us find the closest "landmark number" on the number line. The ones digit is your guide – it tells you which direction to go and which ten is truly your closest neighbor.

Sample questions

1. If you have 47 bolts, what is the nearest ten to estimate your inventory?

✓ 50

○ 40

○ 45

○ 100

Answer: 50 — Since 7 is 5 or greater, we "round up" to the next ten.

2. Round the number 152 to the nearest ten.

○ 160

✓ 150

○ 200

○ 155

Answer: 150 — The digit in the ones place is 2. Since 2 is less than 5, the tens digit stays at 5.

3. What happens when you round 99 to the nearest ten?

○ 90

○ 110

✓ 100

○ 95

Answer: 100 — Rounding the 9 up causes a "carry over" into the hundreds place, making it 100.

Skills in this topic

Round to the nearest ten
Round to the nearest hundred
Estimate sums to the nearest ten or hundred
Estimate differences to the nearest ten or hundred
Determine if an estimate is reasonable

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