← Basic Math

Unit 2: Addition and Subtraction

Joining and taking away made visual — how carrying and borrowing move actual ones, tens, and hundreds around.

5 topics · 19 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

What Addition and Subtraction Really Mean

  1. How it worksHow do you show adding and subtracting as moving along a number line?
  2. Worked exampleFind the total if you have 8 stickers and get 5 more from a friend.
  3. Going furtherWhat happens to a subtraction problem if the number you're taking away is bigger than what you started with?
  4. Ties togetherHow does knowing an addition fact help you solve the matching subtraction fact instantly?
Your turn1 of 2
Maya has 35 stickers and gets 33 more from a friend. How many stickers does she have in total?

Regrouping: Bundling Ten Ones into a Ten

  1. How it worksHow do you bundle ten separate ones together with a rubber band and carry that new ten over?
  2. Going furtherWhat happens when you have to bundle ones into a ten more than once in the same problem?
  3. Ties togetherHow does bundling ten ones into a ten connect to adding two multi-digit numbers together?
Your turn1 of 2
You are counting loose ones on a number chart and reach exactly 10 of them. What do you do with those ten ones?

Adding Multi-Digit Numbers with Regrouping

  1. How it worksHow do you carry a regrouped ten into the tens column when adding two numbers?
  2. Common mistakeWhy isn't 27 plus 15 equal to 312 if you just write the ones sum next to the tens sum?
  3. Worked exampleFind the sum of 58 and 67 using regrouping.
  4. Worked exampleFind the sum of 246 and 178 using regrouping.
  5. Going furtherWhat happens when adding three numbers together causes you to carry a ten more than once?
Your turn1 of 2
A shop sold 67 muffins in the morning and 63 muffins in the afternoon. How many muffins did it sell in all?

Subtracting Across Zeros

  1. How it worksHow do you borrow from the hundreds place when the tens and ones are both zero?
  2. Worked exampleFind the difference between 600 and 274.
  3. Common mistakeWhy isn't the answer to 300 minus 158 found by just writing zero every time you can't subtract?
  4. Ties togetherHow does borrowing across zeros connect to bundling ten ones into a new ten?
Your turn1 of 2
A charity's fundraising goal is 800 dollars. So far they have collected 424 dollars. How much more do they need to reach the goal?

Subtracting Multi-Digit Numbers with Regrouping

  1. Worked exampleFind the difference between 542 and 267 using regrouping.
  2. Common mistakeWhy isn't 63 minus 28 solved by subtracting the smaller digit from the bigger one in each column?
  3. Going furtherWhat happens when subtracting 456 from 1,000 forces you to borrow across three columns in a row?
Your turn1 of 2
A library has 718 books, and 229 of them are checked out. How many books are still on the shelf?