Answer:
 50 inches
Step-by-step explanation:
Since you want the result in inches, you can convert both measures to inches before subtracting.
 8 ft = 8 × 12 in = 96 in
 3 ft = 3 × 12 in = 36 in
so 3 ft 10 in = 36 in + 10 in = 46 in
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The amount of pipe remaining is ...
 96 in - 46 in = 50 in
The plumber will have 50 inches of copper pipe left after the repair.