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Answer:
 10.625 seconds
Step-by-step explanation:
distance = speed · time
We want the total of all distances to be zero.
 (-3.5)(90) +(2.2)(120) +(4.8)t = 0
 -315 +264 +4.8t = 0 . . . . . simplify
 -51 +4.8t = 0 . . . . . . . . . . . simplify more
 -10.625 +t = 0 . . . . . . . . . .divide by 4.8
 t = 10.625 . . . . . . . . . . . . . add 10.625
It will take the scientist 10.625 seconds to reach sea level.