According to question ,
3 pumps working in 2 days = 8 × 2 = 16 hours
It means that they can empty a tank in 16 hours .
3 pumps 1 hour work = 1/16
1 pumps 1 hour work = 1/( 3 × 16 )
4 pumps 1 hour work =4 × 1/( 3 × 16 ) = 1 / 12
Hence , they will empty the tank in 12 hours .
