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
1 · Read
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.
More to read
Adding and subtracting within 1,000: FAQ
A missing number is a covered seat on the bus. In 245 plus something equals 380, the something is the gap between 245 and 380. Hop from 245 to 380 on a number line and measure the hop: 135. Cover the answer, hop it yourself, then uncover to check the gap.
Translating Word Problems into Numeric Equations
Picking a method is picking the right tool. Friendly numbers like 300 and 550 invite quick mental hops, while tangled numbers like 245 and 380 invite blocks or columns. Ask which tool fits before you start digging. The wrong shovel still digs, but the right one works better.
At Home Learning: Solving Two-Step Problems: Word Problems
Two-step puzzles chain two hops into one journey. For 145 plus something plus 200 equals 500, first gather the known walkers: 145 and 200 already cover 345. The gap left to 500 is 155. Solve chains left to right, and park each finished pile before marching on.
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2 · Watch
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Where it sits
Learn first
Optional connections
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
17 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.