if you are using the tank 8 hours a day for 30 days
that is 8*30 = 240 hours
1 tank x tanks
---------- = -------------
12 hours 240 hours
using cross products
1 * 240 = 12 * x
240 = 12 x
divide each side by 12
240/12 = 12x/12
20 = x
You will need 20 tanks
