I am also getting 0.789.
My Approach is:-
A = prob of A is sold = 0.6
B = prob of B is sold. = 0.4
A' = prob of A not sold = 0.4
B' = prob of B not sold. = 0.6
We have total four cases as
A'B'
A'B
AB'
AB
Out of these only last three are valid as per question:-
Given that at least one computer has been sold.
So we eliminate 1st case, since no computer sold in that case.
And question is asking Prob of A is sold for this we will have case 3 and case 4, as in these cases Comp A has been sold.
So, plugging the values we have,
$\frac{0.6 * 0.6 + 0.6* 0.4}{0.6 * 0.6 + 0.6* 0.4 + 0.4*0.4}$
$\frac{60}{76}$
0.789
Please let me know what is wrong in this??