Missing number problems (age 7+)
Solve addition and subtraction problems including missing-number problems, using number facts, place value, and more complex methods
What a learner can do afterwards
- Solve a missing-number problem such as 245 + ? = 380
- Choose an appropriate method (mental, written, or combination) based on the numbers
- Solve multi-step problems combining addition and subtraction of three-digit numbers
The lesson
Some sums have a secret number, shown as a box or a question mark. Your job is to find what is hiding. The trick is simple: use the numbers you already know to work out the missing one.
Zara sees this sum: 245 + ? = 380. She knows the start and the end, so she flips it into a subtraction: 380 − 245 = 135. The missing number is 135.
Addition and subtraction are opposites. If a missing number is in a plus sum, turn it into a minus sum. If it is in a minus sum, turn it into a plus sum.
Ben has 375 stickers. He gives away 120, then he gets 45 more. First subtract: 375 − 120 = 255. Then add: 255 + 45 = 300. Ben now has 300 stickers.
To find a missing number, swap the plus and minus around, then work out the gap.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.