91Ó°ÊÓ

The table gives the categorization of the subjects into four groups.

The probability for a simple event is:

$P\left(E\right)=\frac{\text{Number}\text{of}\text{favorable}\text{outcomes}}{\text{Total}\text{number}\text{of}\text{outcomes}}$

Two or more events can take place one after the other in two manners:

• With replacement: The probability of the subsequent draw remains the same.
• Without replacement: The probability of the subsequent draw changes (the previous draw is not replaced).

Compute the sum row wise and column wise.

Define X as the event that the first subject developed flu and Yas the event that the second subject selected also developed flu.

The number of subjects who had developed flu is 109.

The number of remaining subjects who had developed flu after the first is selected is 108.

The total number of subjects is 1602.

The total number of remaining subjects, after the first is selected, is 1601.

The probability for events X and Y:

$$\begin{gathered} P\left(X\right)=\frac{109}{1602} \\ P\left(Y\right)=\frac{108}{1601} \end{gathered}$$

Thus, the probability that both subjects developed flu is:

$$\begin{gathered} P\left(X\text{and}Y\right)=P\left(X\right)×P\left(Y\right) \\ =\frac{109}{1602}×\frac{108}{1601} \\ =0.00459 \end{gathered}$$

Therefore, the probability that both subjects developed flu is 0.00459.