Question 896278
A and B alternate over a 2 minute period for some quantity of these periods.


{{{(1/20)*1-(1/30)*1}}}, a two minute period.
Same as (1/20-1/30)*p, using p to count the number of these two-minute periods.


Fill 1 tank.
{{{(1/20-1/30)p=1}}}
{{{(3/60-2/60)p=1}}}
{{{(1/60)p=1}}}
{{{p=60}}}, the count of the two-minute alternating time periods.


This means, {{{2*p=2*60=120}}} minutes, 
2 hours.