The total volume of water in the tank after 3 mins 24.75 gallons.
The new volume of water that entered into the tank is calculated as follows;
V₁ = 0.1 L/sec x (3 x 60 s)
V₁ = 18 L
1 liter = 0.264 gallons
18 Liter = ?
= 18 x 0.264
= 4.75 gallons
V = 20 gallons + 4.75 gallons
V = 24.75 gallons
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