Probability

Box 1 contains three cards bearing numbers 1,2,3; box 2 contains five cards bearing cards 1,2,3,4,5; and box 3 contains seven cards bearing numbers 1,2,3,4,5,6,7.A card is drawn from each o the boxes.Let xi be the number on the card drawn from the ith box i=1,2,3.

The probability that x1,x2,x3 are in an arithmetic progression is