(1) There are only two possibilities to pass the qualification examination: 1, and three qualified products are selected for the first time. The probability of this situation is:

c(3,4)( 1/2)^3*( 1/2)^4= 1/64； 2, the first time to draw 4 qualified products, the second time to draw 1 qualified products, the probability of this situation is:

c(4,4)( 1/2)^4*( 1/2)=2/64。 So the total probability of passing the inspection is:

(Note that the power of the last item in the answer is wrong, and 2 should be changed to 3. )

Events A, B, C and D in the topic are only standardized or not, and have little to do with the filling of the final answer of the topic (this depends on the preference of the marking teacher, but generally there are processes and answers with basic full marks)

(2)

The total cost may be 400,500,800.

Q, Q: Where did 400 come from?

A. Answer: A failure means that neither of the two situations described in (1) has occurred. The probability is 1-P (case 1) -P (case 2) =1-c (3,4) (1/2) 3 (1/2)-c (4,4).

Q, where did 500 come from?

A. A: Two sources:

(1) All 4 pieces were qualified in the initial inspection, and 1 piece was qualified in the re-inspection;

(2) All 4 pieces were qualified in the first inspection, and 1 piece was unqualified in the second inspection;

Total probability: p = c (4 4,4) (1/2) 4 * c (1,2) (1/2) =1/6.

Q, Q: Where did 800 come from?

A: As long as you pass the three tests for the first time, you have to spend this money whether you pass the second time or not.

So the probability only needs the first half of p = c (3 3,4) (1/2) 3 (1/2) = 4/16.

Satisfied hope to adopt! ! ! ! Thank you ~ ~ ~