You can find a missing number by undoing the operation. Adding and subtracting undo each other, and so do multiplying and dividing. If the number had two steps done to it, undo the last step first.
The steps, on one like it
What number goes in the box? □ - 386 = 457
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Say it as a story: a number had 386 taken away, and 457 is left.
Saying it aloud shows which operation happened to the box.
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Addition undoes subtraction, so add: 457 + 386 = 843.
Putting back what was taken away gives the starting number.
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Check: 843 - 386 = 457.
Your answer in the box should make the number sentence true.
Answer: 843
The mistake people often make
Using the same operation instead of its inverse, so 457 - 386 = 71 goes in the box.
Undo the operation in the sentence. But if the box is the amount taken away, or the number you divide by, use the same operation instead.
Keep it balanced
Picture a balance. One pan holds □ - 386 and the other holds 457. The pans are level. Add 386 to both pans. The left pan becomes just □, and the right pan becomes 457 + 386 = 843. Doing the same thing to both sides keeps the balance level, so □ = 843.
Words to know
- inverse
- the operation that undoes another one