let Rahul take `6x`(will complete `1/6x` work in 1hour) hours ,Manick will take `3x`(will complete `1/3x` work in 1hour) hours and Sachin will take `2x`hours.(will complete `1/2x` work in 1hour)
`1/6x+1/2x+1/3x=1/4`
=>`x=4` hours
Rahul will take 24 hours, Manick will take 12 hours and Sachin will take 8 hours.