Ankit, Brijesh, and Chirag can complete a task in 30,

Solution

 Total work = LCM of 30, 60, and 90 = 180 units.  Ankit's daily work = 180/30 = 6 units.  Brijesh's daily work = 180/60 = 3 units.  Chirag's daily work = 180/90 = 2 units.  Work done in the first 6 days = 6 × (6 + 3 + 2) = 66 units.  Work done by Brijesh and Chirag together in the next 10 days = 10 × (3 + 2) = 50 units.  Remaining work = 180 - 66 - 50 = 64 units.  Time taken by Chirag to complete the remaining work = 64/2 = 32 days.  Total days Chirag worked = 6 + 10 + 32 = 48 days.