Question 353513
The probability that a set taken is A is 0.40.  Since 3% of these have stereo, the probability that a set taken is A with stereo is {{{0.40*0.03 = 0.012}}}.
Similarly, the probability that a set taken is B with stereo is 
{{{0.40*0.07 = 0.028}}}, and
the probability that a set taken is C with stereo is {{{0.20*0.09 = 0.018}}}.
Since these events are mutually exclusive, or disjoint, the probability that a set taken has stereo is {{{0.012 + 0.028 + 0.018 =0.058}}}.