Answer:
Expected value = 190
Variance = 4000
Explanation:
Let X be the number of the trials until the third success of the bad pump.
This implies that X is a negative binomial distribution having θ = 20% = 0.2.
Now, if for example it will take X trials to use up the three pumps, then the total time is 10 min/trials + extra 10 minutes for the 3 bad pumps
This means the total time is written as;
T = 10X + (10 + 10 + 20)
T = 10X + 40
Mean which is also expected value of X is;
μ_x = 3/0.2 = 15
Variance of X is; σ²_x = 40
Thus;
Mean of T will be;
μ_T = 10μ_x + 40
μ_T = 10(15) + 40
μ_T = 190
Also, variance of T will be;
σ²_T = 10²•σ²_x
σ²_T = 100 × 40
σ²_T = 4000