the probability of having K heads out of N flips can be found with the formula:
[tex]P(N,K)\text{ = }\frac{C^K_N}{2^N}[/tex]the 2 comes from having only two possible outcomes!
So, we need to find P(4,2) + P(4,3) + P(4,4):
[tex]\begin{gathered} P\mleft(4,2\mright)+P\mleft(4,3\mright)+P\mleft(4,4\mright)\text{ = }\frac{1}{2^4}\cdot\text{(}\frac{4!}{2!\cdot2!}\text{ + }\frac{4!}{3!}+\text{ }\frac{4!}{4!}\text{ ) } \\ =\text{ }\frac{6+4+1}{16}\text{ = }0.6875 \end{gathered}[/tex]
