Answer
$\dfrac{1}{4} \text{ or } 0.25$
Work Step by Step
We know that probability of an event is $E$ is given by the formula:
$P(E)=\dfrac{\text{number of favourable outcomes}}{\text{number of all possible outcomes}}.$
The possible outcomes, with $B$=boy and $G$-girl, are:
$\{BBBB,GBBB,BGBB,BBGB,BBBG,GGBB,GBGB,GBBG,BGGB,BGBG,BBGG,BGGG,GBGG,GGBG,GGGB,GGGG\}$
Notice that there are $4$ possible outcomes with $1$ girl and $3$ boys.
There are $16$ possible outcomes in total.
Thus,
$P(\text{$1$ girl and $3$ boys })=\dfrac{4}{16}=\dfrac{1}{4}=0.25$
