So pipe A's filling rate is 1 tank per 16 min or  or 

Therefore C's cross-sectional circumference is 3 times A's cross-sectional
circumference, and since area varies as the square of the circumference,
C's cross-sectional area is 3² or 9 times A's cross-sectional area. 

Since

 therefore:

Pipe C's filling rate is 9 times A's filling rate or 

Since area varies as the square of the circumference. their cross-sectional
areas are in the ratio of 2²:3² or 4:9, and since the rate at which water flows
through a unit cross-sectional area is same for all the three pipes, pipe B's
filling rate is ths of C's filling rate or  or .

B's and C's combined filling rate is 

+ = + =  
 
Since the second tank is twice as big as the first tank (I assume in volume),
it is the same as if they filled 2 tanks the size of the tank that A fills.

We will borrow the equation (rate)(time)=(distance covered), from motion
problems, by replacing "distance covered" by "tanks filled". 

Let t = the number of minutes it will take B and C to fill 2 tanks:

Then (B and C's combined rate)(time) = (2 tanks) 


       t = 2

Multiply both sides by 
    
          t = 2·
          t =  = 2.461538462 or about 2.5 minutes.

Edwin