The Probability Of Twins Being Born Is 1/90, How Do You Solve These Two Problems?

From a statistical point of view, the answer to these questions depends on the confidence interval you want (as well as the issue raised below).

From a mathematical point of view, the "expected value" of the answer to your questions depends on whether "twins being born" constitutes 1 birth or 2. In other words, the probability of being born a twin (from the child's point of view) is different from the probability of twins being born (from the mother's point of view). Ellie82 has apparently assumed "twins being born" = "being born a twin".

Let's assume the probability = 1/90 of "twins being born" means we're saying that the birth of 2 twins = 1 set of twins = 2 births occurs once in 90 (full term, live birth) pregnancies, and the remainder are single-child births. In other words, the probability is that there are 2 twins per 91 births. (If we take triplets, quads, etc. Into account, the number of births in 90 pregnancies will be different from 91.)

A) Expected number of children who are twins in 27 000 births = 2/91*27000 = 593

B) Expected number of sets of twins in 100 000 births = 1/91*100000 = 1099.
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The lesson here is that you have to be very careful to define what the numbers mean, especially in statistics.

Probability Of Twins Being Born = 1/90

A) If during one year there where 27 000 births, how many of these would probably be twins?

Number of twins = Probability of twins x number of births.
= 1/90 x 27000
= 300 twins

Total number of twins = 300

B) How many sets of twins would be born if there where 100 000 births
Number of twin sets = (Probability of twins x number of births)/2

= (1/90 x 100000)/2
= (1111)/2
= 555

Total number twins sets = 555