$[$R$]$ Suppose that $30\%$ of computer users use a Macintosh, $50\%$ use a Microsoft Windows PC

and that $20\%$ use Linux. Also suppose that $60\%$ of the Macintosh users have succumbed to

a computer virus, $80\%$ of the Windows PC users get the virus and $10\%$ of the Linux users

get the virus. A computer user is selected at random and it is found that her computer

was infected with the virus. What is the probability that she is a Windows PC user?