His equation could be written in quadratic form, which is ax^2+bx=c
I'm guessing 90 hours because if you add their hours together it'll be 180 if they work together it'll be like half the time which is 90
First, we should apply the present value of annuity formula. It is

and in this formula

is the present value of annuity factor.
Then, we can find the present value of the annuity by writing that,

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Answer:




Step-by-step explanation:
Given
close
fail to close


First, calculate the value of q
Using complement rule



So, we have:
and 
Solving (a): Fails to close on the 4th attempt
This means that he closes the first three attempts. The event is represented as: p p p q
So, we have:




Solving (b): He closes for the first time on the 3rd attempt
This means that he fails to close the first two attempts. The event is represented as: q q p
So, we have:




Solving (c): First he closes is his 2nd attempt
This means that he fails to close the first. The event is represented as: q p
So, we have:



Solving (d): The first he close is one of his 3 attempts
To do this, we make use of complement rule
The event that he does not close any of his first three attempts is: q q q
The probability is:


The opposite is that the first he closes is one of the first three
So, we have:
--- complement rule




The answer on this question is D