Q. If Ramesh, Suresh and Harish can do a piece of work in 15 days, 10 days and 6 days respectiVelY how long will they take to do it, if all the three work at it together ?

1. 3 days
2. 3 * (9/20) days
3. 3 and half days
4. 3 * (3/20) days