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Three companies $X, Y$ and $Z$ supply computers to a university. The percentage of computers supplied by them and the probability of those being defective are tabulated below

$$\begin{array}{|c|c|c|} \hline \text{Company} & \% \text{ of computers supplied} & \text{Probability of being defective} \\ \hline \mathrm{X} & 60 \% & 0.01 \\ \hline \mathrm{Y} & 30 \% & 0.02 \\ \hline \mathrm{Z} & 10 \% & 0.03 \\ \hline \end{array}$$

Given that a computer is defective, the probability that it was supplied by $Y$ is

1. $0.1$
2. $0.2$
3. $0.3$
4. $0.4$