Answer:
8 hours 32 minutes
Step-by-step explanation:
Working together, the construction rates add. We can use the given relation to write an equation for the total time when the pair work together.
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Let k represent Kirk's time (in hours) to build the wall by himself. Then (k-1) is the time it takes for Jeff to build it. Their working-together rate in "walls per hour" is ...
1/k +1/(k -1) = 1/4
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Multiplying by 4k(k-1), we have ...
4(k-1) +4k = k(k -1)
k² -9k = -4 . . . . . . . . subtract 8k and simplify
k² -9k +20.25 = 16.25 . . . . . add (9/2)² to complete the square
k -4.5 = √16.25 . . . . . . . . . take the square root
k = 4.5 +√16.25 ≈ 8.531129 . . . hours
The fractional hour is ...
0.531129 × 60 min ≈ 31.9 min ≈ 32 min
It would take Kirk about 8 hours 32 minutes to build the wall by himself.
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Additional comment
For most "working together" problems the rates of completing work need to be expressed in jobs per unit time. Usually, the rates are given in terms of time per job, as they are here. That is why reciprocals get involved.
It will take Kirk about 8 hours 32 minutes to build the wall by himself.
Quadratic equations are second-degree algebraic expressions and are of the form ax² + bx + c = 0.
Here, Let k represent Kirk's time (in hours) to build the wall by himself. Then (k-1) is the time it takes for Jeff to build it. Their working-together rate in "walls per hour" is ...
1/k +1/(k -1) = 1/4
Multiplying by 4k(k-1), we have ...
4(k-1) +4k = k(k -1)
k² -9k = -4 . . . . . . . . subtract 8k and simplify
k² -9k +20.25 = 16.25 . . . . . add (9/2)² to complete the square
k -4.5 = √16.25 . . . . . . . . . take the square root
k = 4.5 +√16.25 ≈ 8.531129 . . . hours
The fractional hour is ...
0.531129 × 60 min ≈ 31.9 min ≈ 32 min
Thus, It will take Kirk about 8 hours 32 minutes to build the wall by himself.
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