Ok, first off, the 3mph is garbage, you dont need it. (what is the speed of boat in STILL water)
Secondly, it travels 120 miles total, as it traveled up 60, than back 60.
So, to find out its rate, you would be distance over time

D= 120...... 40
-------------- ------- = About 13.3333 (Repeating 3's) mph
T= 9 ......... 3

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