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On a number line of whole numbers you stand at 4 and try to move 9 steps to the left. What does this show?

Class 6Whole NumbersWhole Numbers
  • AYou land on 5
  • BYou land on 0 and stop there
  • CYou move past 0, so 4 - 9 is not a whole number
  • DYou land on 13

Step-by-Step Solution

Moving left means subtracting, and 4 - 9 = -5. After 4 steps you reach 0, and the remaining 5 steps carry you to the left of 0, off the whole-number line. This is exactly why subtraction is not closed on whole numbers. Landing on 5 comes from subtracting the other way round (9 - 4), and 13 comes from moving right instead of left.
Answer: C) You move past 0, so 4 - 9 is not a whole number

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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