AP ICET 18th Sep 2021 Shift 2 Paper

Pipe 'A' can fill a tank in 9 minutes and pipe 'B' can fill it in 12 minutes. Pipe ' $\mathit{A}$ ' started working and after ' $\mathit{x}$ ' minutes, pipe ' $\mathit{B}$ ' is released. If both of them together filled the rest of the tank in 4 minutes, then $\mathit{x}=$ (in minutes)