Question 365992
A tank can be filled in 4 days using pipe A alone, and in 3 days using both A and B. How many days will take to pipe B to fill the tank?

Let b = number of days it will take pipe B to fill the tank
   1/b= fraction of tank filled up per day using pipe b
   1/4= fraction of tank filled up per day using pipe a

Since a tank can be filled in in 3 days using both A and B
 (1/b +1/4)*3 = 1
  3/b + 3/4   = 1
          3/b = 1/4
            b = 12

It will take 12 days for pipe B to fill the tank