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My approach is THIS :
(a) 1 gallon = 3.785 liters;
(b) 1 trillion = 1000 billions = .
(c) 17 trillions gallons of water = liters of water = cubic meters of water = tons of water. ANSWER
The problem's formulation invites to make a lot of unnecessary calculations and conversions - I chose much simpler way.
On the way, I used conversion
"1 cubic meter of water weights 1 metric ton"
which should be known to everyone who studied Science.