Given that there are 12 microwave ovens i.e., n=12

in that 3 are deffective

then number of good units =9

The probability of good unit is \(\displaystyle={\frac{{{9}}}{{{12}}}}\)

\(\displaystyle={0.75}\)

A vending company has ordered four ofthese units then the probability that: all four units are good is

\(P[X=4]=\frac{\left(\begin{array}{c}9\\ 4\end{array}\right)}{\left(\begin{array}{c}12\\ 4\end{array}\right)}\)

Probability of exactly two are good is

\(P[X=2]=\frac{\left(\begin{array}{c}9\\ 2\end{array}\right)}{\left(\begin{array}{c}12\\ 4\end{array}\right)}\)

Probability of at least two are good is

\(\displaystyle{P}{\left[{x}\geq{2}\right]}={P}{\left[{X}={2}\right]}+{P}{\left[{X}={3}\right]}+{P}{\left[{X}={4}\right]}\)

\(=\frac{\left(\begin{array}{c}9\\ 2\end{array}\right)}{\left(\begin{array}{c}12\\ 4\end{array}\right)}+\frac{\left(\begin{array}{c}9\\ 3\end{array}\right)}{\left(\begin{array}{c}12\\ 4\end{array}\right)}+\frac{\left(\begin{array}{c}9\\ 4\end{array}\right)}{\left(\begin{array}{c}12\\ 4\end{array}\right)}\)

Solve the above we get the required answer