How many whole numbers have exactly two digits?

Class 6Whole NumbersWhole Numbers

A89
B99
C100
D90

Step-by-Step Solution

The two-digit whole numbers run from 10 up to 99. Count them as 99 - 10 + 1 = 90. Answering 89 forgets to add the 1 after subtracting, and 99 counts every number from 1 to 99 instead of only the two-digit ones.

Answer: D) 90

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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