Solutions - 5070-05

(a)

Out of $7$ trials, we choose $3$ of them to be heads.

$$\left(\genfrac{}{}{0px}{}{7}{3}\right)=35$$

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(b)

Out of 7 trials, we choose at least 3 of them to be heads.

$$\sum _{k=3}^7\left(\genfrac{}{}{0px}{}{7}{k}\right)=99$$