Answer:
2 2/3 hours
Step-by-step explanation:
Before it was 5 hours for 1 guitar, or 5 hours/guitar
Now it is 7 guitars in 3 hours or 7/3 hours/guitar
7/3 = 2 1/3
5 hours - 2 1/3 hours = 15/3 hours - 7/3 hours = 8/3 hours = 2 2/3 hours
The reduced the time it takes to build 1 guitar by 2 2/3 hours.