A swimming pool is fitted with three pipes, the 1st two pipes working simultaneously can fill the pool in the same as the 3rd pipe alone

A swimming pool is fitted with three pipes, the 1st two pipes working simultaneously can fill the pool in the same as the 3rd pipe alone, the 2nd pipe alone fills the pool 5 hours faster than 1st pipe and 4 hours slower than 3rd pipe. In what time 2nd and 3rd pipe together fill the pool?

Sol: