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On a number line an arrow starts at 12, points to the right and ends at 30. Which whole-number sentence does this arrow show?

Class 6Whole NumbersWhole Numbers
  • A30 - 12 = 18
  • B12 + 18 = 30
  • C12 + 30 = 42
  • D

Step-by-Step Solution

The arrow begins at 12 and moves right, so it models an addition that starts at 12. Its length is 30 - 12 = 18 steps, giving 12 + 18 = 30. The sentence 30 - 12 = 18 is correct arithmetic but it describes an arrow starting at 30 and pointing LEFT.
Answer: B) 12 + 18 = 30

Key idea: Closure: when an operation 'stays inside' the whole numbers

Adding two whole numbers always gives a whole number — addition is **closed** on whole numbers. Multiplying two whole numbers always gives a whole number — multiplication is **closed** too. But subtraction is **not** closed: 5 − 7 = −2, which is not a whole number. Division is also not closed: 7 ÷ 2 = 3.5, not a whole number. Closure is the property that lets us trust an operation never to take us 'outside' the world of whole numbers.

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