It takes Preston 14 hours to repair the broken pipes.
Given Information
Cost of service call charged by Preston, the plumber, S = $50
Preston charges $5/hour.
Preston's charge per hour, P = $5
The total bill, B = $120
We have to calculate the number of hours Preston take to repair the broken pipes.
Calculating the Number of Hours
Total service charge of Preston = Total bill - Service Call Charge
= B - S
∴ Number of hours Preston took to mend the broken pipe is given by,
H = (B - S)/P
H = $(120 - 50)/$5
H = 70/5
H = 14 hours
Therefore, Preston took 14 hours to repair the pipes.
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