Question 1098414
Look at the possible outcomes, there are {{{2^4}}} or {{{16}}} of them.
I will also count the number of boys in each outcome.
GGGG=0
GGGB=1
GGBG=1
GGBB=2
GBGG=1
GBGB=2
GBBG=2
GBBB=3
BGGG=1
BGGB=2
BGBG=2
BGBB=3
BBGG=2
BBGB=3
BBBG=3
BBBB=4
So out of the 16 outcomes, count the number with the given number of boys,
{{{B=0}}},{{{1}}}
{{{B=1}}},{{{4}}}
{{{B=2}}},{{{6}}}
{{{B=3}}},{{{4}}}
{{{B=4}}},{{{1}}}
So the probability of having three boys and one girl are,
{{{P(B=3)=4/16=1/4}}}