Interview Question

Technology Summer Intern Interview


Susquehanna International Group (SIG)

a family have 2 kids. if you have seen 1 girl , what is the proberbl the other kids is a boy

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10 Answers


chainsaw is right - the question is subtle. Outcome space is (b1,b2), (b1,g2), (g1,b2) and (g1,g2). Now - if you HAVE SEEN the girl (specific girl let's say girl 1) that removes outcomes (b1,b2)&(b1,g2) leaving us with p=1/2 of choice between outcomes (g1,b2) and (g1,g2). If you KNOW (was told) that one is a girl, but do not know which one, than only (b1,b2) is removed from outcome space and the chance is 2/3.

Yury on


answer is 1/2 - the question is very subtle...

chainsaw on


isnt it 1/2? BG (boy and girl) and GB (girl and boy) are the same. order is not important. or am i missing something?

xts on


BG; GB; GG: 2/3 other child is a boy? Bayes' Theorem?

marty holah on


2/3 is right ... the sample space with 2 children is BB GB BG GG. given u know one is a girl therefore cross off the BB option. hence we have either GB or BG or GG as for BG GB GG GG well think of dice, u dont get 2 different 1,1 's when rolling 2 dice

Anonymous on


It is 1/2 and question is really subtle. 2/3 is the probability of the other one is a boy given one of the 2 kids is girl AND WE DO NOT KNOW WHICH ONE IS THE GIRL. However this one, you have seen one girl, you know which one is the girl and the sex of the other one is independent so 1/2

Anonymous on


The gender of the two children is an entirely independent event. There is no nonrandom agent specified here. The Monty Hall 2/3 applies only because it's given that Monty will NOT open the prize door and so he is not a random agent. Probability the other child is a boy is 1/2.

wastedTime on


Why not BG, GB, GG, GG? 2/4?

Samsonite on


I think 2/3 is wrong. Lets say the first kid is Girl which we have seen. the other kid is going to be a boy or a girl. So - probability is 50%

JD on


Actually, you are right., should be as explained- 2/3 chance.

JD on

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