Suppose that 9% of births are twins. If we assume no other multiple births (triplets, etc.), what percentage of the population are twins?

Question

Hint 1: The answer is not 9% or 18%. Hint 2: Do not confuse the given percentage of births with the percentage of the population. Hint 3: Use 100 births to calculate the answer.

Answer 1

First find out how many births were twins and how many were singular births:

9% twins of 100 births = 9 twin births

that leaves 91 single births (100-9)

 

Next, find the total number of people:

9 twin births * 2 people per birth = 18 people

91 single births * 1 person per birth = 91 people

 

Then, find the percentage that are twins:

18 + 91 = 109 total people

18 twins / 109 total people = 16.51....%