p(x=2)=3/8*6/8=9/32

p(x = 3)= 3/8 * 2/8 * 7/8 = 2 1/256

p(x = 4)= 3/8 * 2/8 * 1/8 * 1 = 3/256

p( 1 & lt； x & lt=3)=253/256

The reason for the first smoke is that the first smoke is red (a- 1). You didn't calculate the probability of drawing red at first. Just multiply your formula by the probability of failure before. Probability can be calculated step by step, and a formula is easy to make mistakes.