What I gather from the question is that has second moment and variance , and you're asked to find the expectation and variance of the random variable .
From the given second moment and variance, we find the expectation of :
Expectation is linear, so
Using the same variance identity, we have
and
so that
Alternatively, we can use the identity
Answer is D because i add 21 and 27 and got 48 then divied by the total number sandwiches and got .80
Answer:
79.77%
Step-by-step explanation:
P(X≥120) = normalcdf(120,1e99,130,12) = 0.7976716754 ≈ 0.7977 = 79.77%
Therefore, about 79.77% of operations last at least 120 minutes