How many positive integers less than 100 have at least one digit that is a 9? (let t be the set of positive integers less than 100 with a 9 in the ten's place. let o be the set of positive integers less than 100 with a 9 in the one's place. now determine t ∪ o).

19

Step-by-step explanation:

Let T be the set of positive integers less than 100 with a 9 in the ten's place.

T = {90, 91, 92, 93, 94, 95, 96, 97, 98, 99}

n(T) = 10

Let O be the set of positive integers less than 100 with a 9 in the one's place.

O = {9, 19, 29, 39, 49, 59, 69, 79, 89, 99}

n(O) = 10

The common element is 99.

The union of both sets is

Therefore, 19 positive integers are less than 100 those have at least one 9.

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step-by-step explanation:

the answer is a if my teacher taught me correct