Home/Second Grade Math/Lesson 128
K-5 friendly lesson
One small step at a time.Read the idea, try the activity, and celebrate each win as you go.

Subtract 3-Digit Models (With Regrouping!)

πŸ”¨ Break out the sledgehammer! You don't have enough Ones cubes... so you have to smash a Rod. You don't have enough Rods... so you have to smash a giant Square Flat!

Do this: Read the concept below, then try the quiz or activity.

Lesson 128 of 186

Concept

Here is exactly what "Borrowing" looks like in the real world.

🎯 The Double Smash Rule:
Problem: 312 - 145 = ?
* Start: 3 Flats, 1 Rod, 2 Cubes.
1. Subtract 5 Cubes? Can't! I only have 2. 
   Smash a Rod! (Take the 1 Rod, smash it to 10 cubes). Now I have 0 Rods, and 12 Cubes.
   Cross out 5 cubes. (12 - 5 = 7 left).
2. Subtract 4 Rods? Can't! I only have 0!
   Smash a Flat! Take 1 Giant Flat and smash it into 10 Long Rods! Now I have 2 Flats and 10 Rods.
   Cross out 4 Rods! (10 - 4 = 6 left).
3. Subtract 1 Flat: Cross out 1 flat. (2 - 1 = 1 left).
4. Answer: 1 Flat, 6 Rods, 7 Cubes. 167!

Try it

Smash to survive!

Instructions: Imagine smashing the block to get 10 smaller pieces.

Problem 1: 451 - 138 = ? * Cubes (1 - 8). Smash a rod! (Make 11 cubes). 11 - 8 = 3. * Rods: We smashed one, so there are 4 left. 4 - 3 = 1. * Flats: 4 - 1 = 3. * Answer: ___

Problem 2: 524 - 162 = ? * Cubes: 4 - 2 = 2. (Safe! No smashing needed). * Rods: 2 - 6. Smash a Flat! Make 12 Rods. 12 - 6 = ___ * Flats: (We smashed one, so 4 left). 4 - 1 = ___ * Answer: ___

Problem 3: 315 - 182 * Answer: ___

Complete & Continue β†’