Estimation Experts: Making Smart Maths Guesses with Rounding!

Hello Maths Detectives! Did you know that sometimes in maths, getting a super-quick “nearly right” answer is just as helpful as getting the exact one? This clever skill is called Estimation, and it’s like making a really smart, sensible guess. One of the best ways to estimate is by rounding numbers to the nearest 10 or 100 before you add or subtract. Let’s learn how!

What is Rounding? Your Tool for Estimation!

Rounding makes numbers simpler and easier to work with in your head.

Rounding to the Nearest 10:

• Look at the ones digit.
• If the ones digit is 5 or more (5, 6, 7, 8, 9), you round up to the next ten. (e.g., 27 rounds up to 30; 45 rounds up to 50)
• If the ones digit is 4 or less (4, 3, 2, 1, 0), you round down to the previous ten (the ones digit becomes a 0). (e.g., 23 rounds down to 20; 41 rounds down to 40)

Rounding to the Nearest 100:

• Look at the tens digit (and the ones too, but the tens digit is the main guide here!).
• If the tens digit is 5 or more (meaning the number is 50, 51,… up to 99 in the tens and ones part), you round up to the next hundred. (e.g., 270 rounds up to 300; 452 rounds up to 500)
• If the tens digit is 4 or less (meaning the number is 49, 48,… down to 00 in the tens and ones part), you round down to the previous hundred (the tens and ones digits become 0s). (e.g., 230 rounds down to 200; 418 rounds down to 400)

How to Estimate Using Rounding

Estimating an Addition Answer (e.g., 243 + 187 by rounding to nearest 100)

• Step 1: Round the first number. 243 rounded to the nearest 100 is 200 (because 43 is less than 50).
• Step 2: Round the second number. 187 rounded to the nearest 100 is 200 (because 87 is more than 50).
• Step 3: Add your rounded numbers. 200 + 200 = 400.
• So, our estimated answer for 243 + 187 is about 400. (The exact answer is 430, so 400 is a good estimate!)

Estimating a Subtraction Answer (e.g., 58 − 23 by rounding to nearest 10)

• Step 1: Round the first number. 58 rounded to the nearest 10 is 60 (because 8 is 5 or more).
• Step 2: Round the second number. 23 rounded to the nearest 10 is 20 (because 3 is 4 or less).
• Step 3: Subtract your rounded numbers. 60 − 20 = 40.
• So, our estimated answer for 58 − 23 is about 40. (The exact answer is 35, so 40 is a pretty close estimate!)

You can choose to round to the nearest 10 for a closer estimate, or to the nearest 100 for a quicker, rougher estimate with bigger numbers.

Practice Your Estimation Skills! (18 Questions)

Ready to become an estimation expert? For each problem below, round the numbers to the nearest 10 OR 100 (the question might tell you which, or you can choose!). Then, add or subtract your rounded numbers to find your estimated answer.

(Your web app with the 18 questions will go here. The questions should provide addition/subtraction sums and ask for an estimated answer via rounding.)

Why is Estimation Such a Handy Skill?

• Quick Checks: It helps you quickly check if your exact answer to a problem is sensible or way off.
• Real-World Maths: In shops, you can estimate the total cost of items. When cooking, you might estimate ingredient amounts.
• Saves Time: Sometimes you don’t need an exact answer, just a good idea of the amount.
• Builds Number Sense: It makes you really good at understanding how big numbers are and how they relate to each other.

Tips for Grown-Ups: Developing Estimation Skills

Estimation is a vital mathematical skill that involves judging whether an answer is reasonable. Rounding is a common strategy for estimation. It’s important for children to understand that an estimate is not an exact answer, but a close approximation.

• Master Rounding First: Ensure children are confident with rounding numbers to the nearest 10 and 100 before applying it to estimation in calculations.
• Emphasise “About” or “Roughly”: Use language like “So the answer will be about…”, or “It’s roughly…” to reinforce that it’s not an exact calculation.
• Compare Estimates to Exact Answers: After estimating, calculate the exact answer (if appropriate for the child’s skill level) and discuss how close the estimate was. This helps them see the value and limitations of estimating.
• Real-Life Opportunities: Point out estimation in everyday situations: “About how many sweets are in this jar if we round?” “The journey is 187 miles, then another 121 miles. Roughly how far is that in total if we round to the nearest 100?”
• Discuss When to Round to 10 vs. 100: Talk about how rounding to the nearest 10 often gives a closer estimate but rounding to 100 can be quicker for very large numbers or when a rougher idea is sufficient.