Let X1, X2, and X3 represent the times necessary to perform three successive repair tasks at a service facility. Suppose they are normal random variables with means of 50 minutes, 60 minutes, and 40 minutes, respectively. The standard deviations are 15 minutes, 20 minutes, and 10 minutes, respectively. a Suppose X1, X2, and X3 are independent. All three repairs must be completed on a given object. What is the mean and variance of the total repair time for this object.

We have that the sum of two numbers  is 9

this can be written as:

x + y  = 9

where x is the larger number.

Now we want to write:

"the difference between one more than the larger number and twice the smaller number"

First, remember that the difference between A and B is:

A - B

Then "the difference between one more than the larger number and twice the smaller number"

is:

"one more than the larger number" = ( x + 1)

"twice the smaller number" = 2*y

the difference between these is:

(x + 1) - 2*y

Now we can simplify:

We know that:

x + y = 9

then:

y = 9 - x

replacing that in the equation:

(x + 1) - 2*y

we would get:

x + 1 - 2*(9 - x)

x + 1  -18 + 2x

(x + 2x) + (1 - 18)

3x - 17

This means that we can write:

"the difference between one more than the larger number and twice the smaller number"

as: 3x - 17