Answer
See below.
Work Step by Step
We know that probability$=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}.$
Let $B$ represent a boy, $G$ a girl, then the sample space is: $X=\{BBBB,BBGB,BGBB,GBBB,GGBB,GBGB,BGGB,GGGB, BBBG,BBGG,BGBG,GBBG,GGBG,GBGG,BGGG,GGGG\}$.
The good outcomes are: $X=\{GGBB,GBGB,GBBG,BGGB,BGBG,BBGG\}$
Hence here probability$=\frac{|X|}{|S|}=\frac{6}{16}=0.375.$