A tank holds 15,000,000,000,000,000,000,000 gallons. Water fills the tank 2gallons the 1st minute and doubles each minute. How long will it take to fill the tank?

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if 2 gallons take 1 min then 15000000000000000000000 needs:
(15000000000000000000000 * 1)/2=7.5* 10^12 min