Given:
The 2 boy selected out of 10 then boy selecting is:
[tex]=^{10}C_2[/tex]Then favorable outcome is:
[tex]\begin{gathered} =^{10}C_2 \\ =\frac{10!}{(10-2)!2!} \\ =\frac{10\times9\times8!}{8!\times2\times1} \\ =\frac{90}{2} \\ =45 \end{gathered}[/tex]Total outcome is:
[tex]\begin{gathered} =^{21}C_2 \\ =\frac{21!}{(21-2)!2!} \\ =\frac{21\times20\times19!}{19!\times2\times1} \\ =\frac{21\times20}{2} \\ =210 \end{gathered}[/tex]Probability is define as:
[tex]\begin{gathered} P(a)=\frac{\text{ favorable outcome}}{\text{ total outcome}} \\ P(a)=\frac{45}{210} \\ =0.2142 \end{gathered}[/tex]