Subtraction Detectives: Cracking the “Crossing the Tens” Case!

Hello Super Solvers! Are you ready for a subtraction mission that’s a little more challenging but super rewarding? Today, we’re going to learn how to mentally subtract a single-digit number from a 3-digit number when we have to cross over the tens boundary. It’s like when the ones digit in your big number is smaller than the number you want to take away – for example, trying to solve 452 take away 5. Let’s investigate!

How to Subtract Across the Tens (e.g., 452 − 5)

When the ones digit of the big number (the minuend) is smaller than the digit you’re subtracting, we can’t just subtract the ones directly. We need a clever mental strategy!

Let’s break down 452 − 5:

Method 1: Subtracting to the Nearest Ten First

• Look at 452. The ones digit is 2. We want to subtract 5.
• First, let’s take away enough to get to the nearest friendly ten below 452. To get from 452 down to 450, we need to subtract 2. (So, 452 − 2 = 450)
• We wanted to subtract a total of 5, and we’ve already subtracted 2. How much more do we need to subtract? (5 − 2 = 3 more)
• Now, subtract that remaining 3 from 450. (450 − 3 = 447)
• So, 452 − 5 = 447!

Method 2: Thinking About “Borrowing” (Mental Version)

• In 452, we can think of the ’52’ part. We need to take 5 from 2 in the ones place, but we can’t!
• So, we mentally ‘borrow’ a ten from the 5 tens. That leaves 4 tens (which is 40).
• The ‘borrowed’ ten joins the 2 ones, making it 12 ones.
• Now we can do 12 ones − 5 ones = 7 ones.
• We still have the 4 hundreds and the 4 tens (that were left).
• So, we have 4 hundreds, 4 tens, and 7 ones: 447.
• Therefore, 452 − 5 = 447!

Choose the mental method that makes the most sense to your amazing brain!

Challenge Time: 18 “Crossing the Tens” Subtraction Problems!

Now it’s your turn to be a subtraction detective! Here are 18 problems where you’ll need to use your “crossing the tens” skills. Read each one carefully, work it out in your head, and then check your solution.

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

Why is Mastering This Subtraction Important?

• Unlocks More Maths: This skill is key for solving lots of different subtraction problems.
• Boosts Problem-Solving: It teaches you to think flexibly with numbers.
• Real-Life Use: Useful for working out change, differences in scores, or sharing things out.
• Makes You a Maths Whizz: Being able to do this mentally is super impressive!

Tips for Parents & Teachers

This activity helps children develop mental strategies for subtracting a single-digit number from a 3-digit number where regrouping (or “borrowing”) from the tens place is required (e.g., 452 − 5).

• Decomposition Strategy: Encourage breaking down the subtraction, e.g., for 452 − 5, subtract 2 to get to 450, then subtract the remaining 3. This is often the most intuitive mental method.
• Visual Aids: Number lines (hopping back past a ten) or even quickly sketching base-ten blocks can help children visualize the regrouping process initially.
• Verbalise the Steps: Have children explain their thinking aloud. This reinforces their understanding and helps identify any misconceptions.
• Consistent Practice: Regular, short bursts of practice are more effective than infrequent long sessions.