Question 946485
Note that independent events are not affected by previous events.
therefore,
Probability of an event happening = Number of ways it can happen / Total number of outcomes
lets look at the total number of outcomes with the first child a boy,
(boy, girl, boy), (boy, boy, girl), (boy, boy, boy), (boy, girl, girl) 
now,
the probability of exactly 2 boys, given that the first child is a boy = 2/4 = 1/2