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Find a missing number in subtraction

Subtraction equations with whole numbers within 20 can be interpreted as related quantities, allowing the unknown to be the starting amount, the amount taken away, or the difference; the relationship can be found by reasoning with the corresponding addition fact, a number line, or a part–part–whole model. This understanding distinguishes the roles of the minuend, subtrahend, and difference and does not extend to negative numbers, decimals, larger multi-digit numbers, or generalized algebraic equations.

Detailed Explanation: Find a missing number in subtraction

In a subtraction equation, the numbers show:

  • Starting amount: how many there were at first
  • Amount taken away: how many were removed
  • Difference: how many are left

Use the related addition fact to find the missing number.

Example:

□−7=5\Box - 7 = 5
  1. The equation says: “A starting number had 7 taken away, and 5 were left.”
  2. Think of the related addition fact: start with what was taken away and add what was left.
7+5=127 + 5 = 12
  1. So the missing starting number is 1212.
  2. Check your answer:
12−7=512 - 7 = 5

Therefore,

12−7=5\boxed{12 - 7 = 5}

Learn by doing: Find a missing number in subtraction

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    Subtraction - Missing Value, Within 20

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