The probability that the first three times were not B is:

There are five possibilities for the first time, four for the second time, three for the third time, and the remaining five times are arranged from five;

There is always an 8-in-8 arrangement.

P=(5*4*3)*(5！ )/8! =5/28;

Another understanding is that the numerator is an arrangement of five threes and the denominator is an arrangement of eight threes.

The probability that at least one of the products tested in the first three times is B is1-5/28 = 23/28;

X may be 1, 2, 3, 4; p 1 = 5/8； Said the first time I drew a nail.

P2 = 3/8 * 5/7； It means the first time to draw B, and the second time to draw A..

P3 = 3/8 * 2/7 * 5/6；

P4=3/8*2/7* 1/6*5/5

E=3/2