I think the answer to this question is option D
First you take the 20 hours it took for the 3 service calls... 20/3=6.67. Then you multiply 6.67X7.5=50.025 hours.
For number 1. it is wrong. It is fresh water. Ocean water is salt water. I don't know the rest, sorry. I will come back if I find out though! :D
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4 12 18
:) fun little riddle
