Let x be the rate of work  of the first tank, in "job per minute", which is "the tank volume per minute".


Then the rate of work of the second tank is    "the tank volume per minute".


In one minute, two pumps fill    of the tank volume.


It means that


    x +  = .


Multiply both sides of this equation by 216.  You will get


    216x + 36x = 1,

    252x = 1

    x    = .


Hence, the first pump will fill the tank in 252 minutes, working alone.    ANSWER

The first, faster pump works as productively, as 6 slower pumps.


So, the given info, if reformulated, says that 6+1 = 7 slower pumps will fill the tank in 216 minutes.


Hence, one slower pump will do it in 7*216 minutes.


The faster tank will do it 6 times faster, i.e. in   = 7*36 = 252 minutes.


You got the same answer as above.