Expert Tens Subtraction: Crossing the Hundreds Barrier!

Hello Maths Champions! You’ve become skilled at subtracting tens. Now, get ready for a mental workout where we subtract multiples of 10 (like 20, 30, 40) from 3-digit numbers, and this time we’ll be crossing over the hundreds boundary! This means the hundreds digit will change. It’s a bit like being a number detective and finding out where the numbers go! Let’s investigate a sum like 622 take away 40.

How to Subtract Tens Across the Hundreds (e.g., 622 − 40)

When the number of tens you’re subtracting is more than the tens digit in your starting number, you’ll need to “dip into” the hundreds.

Let’s break down 622 − 40:

Revised Method 1: Subtracting in Parts to Cross the Hundred

• Our starting number is 622. We want to subtract 40.
• Step 1: How many tens can we take away easily from the ’22’ part to get to the solid hundred (600)? We can take away 2 tens (or 20).
  • So, 622 − 20 = 602.
• Step 2: How many tens do we still need to subtract? We wanted to subtract 40, and we’ve subtracted 20.
  • 40 − 20 = 20 more tens to subtract.
• Step 3: Subtract the remaining tens from our new number. Now we do 602 − 20.
  • Think of 602 as 60 tens and 2 ones. Subtract 2 tens from 60 tens, which is 58 tens. The 2 ones are still there.
  • So, 602 − 20 = 582.
• Therefore, 622 − 40 = 582!

Method 2: Thinking About “Borrowing” from the Hundred

• In 622, we have 6 hundreds, 2 tens, and 2 ones. We want to subtract 4 tens.
• We can’t take 4 tens from 2 tens directly. So, we “borrow” 1 hundred from the 6 hundreds. This leaves 5 hundreds.
• That borrowed 1 hundred is the same as 10 tens. We add these 10 tens to the 2 tens we already have: 10 tens + 2 tens = 12 tens.
• Now we can subtract the 4 tens: 12 tens − 4 tens = 8 tens.
• We still have 5 hundreds, and the 2 ones haven’t changed.
• So, we have 5 hundreds, 8 tens, and 2 ones: 582.
• Therefore, 622 − 40 = 582!

Choose the method that clicks best in your brain!

Brain Challenge: 18 “Crossing the Hundreds with Tens” Subtractions!

It’s time to flex those mental maths muscles! Here are 18 subtraction problems. You’ll need to subtract multiples of 10 and cross the hundreds boundary. Take it step-by-step.

(Your web app with the 18 questions will go here.)

Why is This Tricky Tens Subtraction So Important?

• Builds Advanced Number Sense: You truly understand how hundreds and tens work together.
• Prepares for Complex Maths: This skill is vital for bigger subtractions and even division.
• Shows Maths Resilience: Tackling and solving these problems builds your “can-do” maths attitude!
• Real-World Application: Useful when dealing with larger amounts of money or measurements where you need to subtract amounts like £40 from £622.

Guidance for Grown-Ups: Navigating This Challenge

This activity focuses on the mental subtraction of multiples of 10 from a 3-digit number, specifically requiring regrouping from the hundreds place (e.g., 622 − 40 = 582; 315 − 30 = 285).

• Reinforce Place Value Exchange: The concept that 1 hundred can be exchanged for 10 tens is crucial. Base-ten blocks can be invaluable for demonstrating this visually if children struggle.
• Break It Down: Encourage strategies like subtracting to the previous hundred first, then subtracting the remainder (Revised Method 1).
• Number Lines: Jumping back on a number line, showing the hop over the hundred, can aid understanding.
• Verbalise Thinking: Ask children to explain their steps. This helps them process the logic and allows you to identify any sticking points.