A bag contains 10 different balls. Five balls are drawn simultaneously and then replaced and then seven balls are drawn. If the probability that exactly three balls are common to the two draws is p, then the value of 8p is ________.

Number of ways of drawing 7 balls (second draw)=10C7For each set of 7 balls of the second draw, 3 must be common to the set of 5 balls of the first draw, i.e., 2 other balls can be drawn in 3C2 ways.Thus, for each set of 7 balls of the second draw, there are 7C3×3C2 ways of making the first draw so that there are 3 balls common.∴ Probability of getting 3 balls in common = 7C3×3C2 10C7                                                                      =78