p(x-3)=15C^3(0.5)3(0.5)15-3

p(x-3)=15C^3(0.5)3(0.5)15-3

$\begin{array}{|rcll|} \hline && 15C_{3}(0.5)^{3}(0.5)^{15-3} \quad & | \quad 15C_{3} = \frac{15}{3}\cdot \frac{14}{2}\cdot \frac{13}{1} = 5\cdot 7 \cdot 13 = 455 \\ &=& 455 \cdot (0.5)^{3}(0.5)^{15-3} \\ &=& 455 \cdot(0.5)^{3+15-3} \\ &=& 455 \cdot(0.5)^{15} \\ &=& 455 \cdot(\frac12 )^{15} \\ &=& 455 \cdot \frac{1}{2^{15}} \\ &=& \frac{455}{2^{15}} \\ &=& \frac{455}{32768} \\ &=& 0.01388549805 \\ \hline \end{array}$