Pipes A and B can fill a tank in 36 hours and 48 hours, respectively. Both pipes are opened together for 9 hours and then A is closed. Pipe B alone will fill the remaining part of the tank now in:

Pipe A can fill a tank  in 36 hours so in one hour A can fill $$\dfrac{1}{36}$$ part of the tank

Pipe B can fill  a tank in 48 hours so in one hour  B can fill $$\dfrac{1}{48} $$ part of a tank

then, in one, our A and B can fill part of tank = $$\dfrac{1}{36}+\dfrac{1}{48}$$

$$\Rightarrow \dfrac {4+3}{144}$$

$$\Rightarrow \dfrac{7}{144}$$

in 9 hours A and B can fill part of tank = $$ 9 \times \dfrac{7}{144}$$

$$\Rightarrow \dfrac{63}{144}$$ 

Remaining part of tank = $$ 1 - \dfrac{63}{144} $$

$$\Rightarrow \dfrac{144-63}{144}$$

$$\Rightarrow \dfrac{81}{144}$$ 

After it A is closed and B is open 

hence B can fill remaining tank in hours = $$\dfrac {81}{144}\times 48$$

$$\Rightarrow 27$$ hours Ans