Question 321016
The rate for 1st pipe is (1 tank)/66 min
The rate for 2nd pipe is (1 tank)/55 min
Their rates of filling the tank with both open
is the sum of the 2 rates working alone
Let {{{x}}} = time to fill with A&B both open
{{{1/x = 1/66 + 1/55}}}
Multiply both sides by  {{{330x}}}
{{{330 = 5x + 6x}}}
{{{11x = 330}}}
{{{x = 30}}}
It takes 30 min with both pipes open
Note that the liters of water in a full tank is not needed to solve