Brain Teasers
Number of Siblings
Probability
Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.Probability
Ten-year-old Bert Sampson lives in the town of Springvale, where, out of all households:
2% include five children
7% include four children
14% include three children
31% include two children
16% include one child
30% do not include any children
What are the chances that Bert lives with two sisters and no brothers?
(Consider all children living in any given house as siblings).
2% include five children
7% include four children
14% include three children
31% include two children
16% include one child
30% do not include any children
What are the chances that Bert lives with two sisters and no brothers?
(Consider all children living in any given house as siblings).
Answer
6.646%Since Bert is a child, he cannot possibly live in a household with no children. Therefore there is a (14*3)/(16+31*2+14*3+7*4+2*5), or 26.582% chance that he lives with two other children (i.e. in a household with three children).
There is a 50% chance that each of these other children is a girl, so there is a 25% chance that they are both girls.
Therefore, the total chance that Bert lives with two sisters is 25% of 26.582%, or 6.646%.
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