The given details are,
Anil can do a piece of work in 5 days
Work done by Anil in 1 day = 1/5
Ankur can do same work in 4 days
Work done by Ankur in 1 day = 1/4
Work done by both in 1 day = 1/5 + 1/4
= (5+4)/20 (by taking LCM for 5 and 4 which is 20)
= 9/20
∴ Total work done together is 1/(20/9) = 20/9 = 2 2/9 days.