The total installations they would have completed is 64 installations
From the question, we are to determine how many total installations they would have completed
If the technician can complete an installation in 1.5 hours
Then,
In 60 hours, the technician will complete [tex]\frac{60}{1.5}[/tex] installations
[tex]\frac{60}{1.5}[/tex] = 40
Therefore, the technician would have completed 40 installations
If it takes the less experienced colleague 2.5 hours to complete an installation
Then,
In 60 hours, the less experienced colleague will complete [tex]\frac{60}{2.5}[/tex] installations
[tex]\frac{60}{2.5}[/tex] = 24
Therefore, the less experienced colleague would have completed 24 installations
Hence, the total installations they would have completed is 40 + 24 = 64 installations
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