12. Suppose we have four boxes A,B,C and D containing coloured marbles as given below:
One of the boxes has been selected at random and a single marble is drawn from it. If the marble is red, what is the probability that it was drawn from box A ?
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Let R be the event of drawing red marble.
Let respectively denote the event of selecting box A, B, C.
Total marbles = 40
Red marbles =15
Probability of drawing red marble from box A is