Answer: The answer is 30%.
Step-by-step explanation: Given that there is a group of 10 light users and 10 heavy users. We are to find the probability that exactly 3 of the 20 users are HIV positive.
We have the following four possible cases -
(i) All 3 are light users.
(ii) 1 is a light user and 2 are heavy users.
(iii) 2 are light users and 1 is a heavy user.
(iv) All 3 are heavy user.
Since there is a 45% chance of a light user to be HIV positive and 55% chance of a heavy user to be HIV positive, so the required probability is given by
[tex]p\\\\=\dfrac{3\times 0.45+1\times 0.45+2\times 0.55+2\times 0.45+1\times 0.55+3\times 0.55}{20}\\\\=\dfrac{6}{20}\\\\=0.3\\\\=30\%.[/tex]
Thus, the probability is 30.
