Ramesh alone can bind a consignment of registers in 28 days, Suresh alone in 21 days and Vikas alone in 42 days. Ramesh and Suresh start together; at the end of the sixth day Ramesh is moved to another branch and Vikas takes his place beside Suresh. How many days does the whole consignment take?

Take the consignment as 84 units: Ramesh 3/day, Suresh 4/day, Vikas 2/day. In six days Ramesh and Suresh bind $6\times 7=42$ units, leaving 42. Suresh and Vikas then manage 4 + 2 = 6 units a day, so they need 42 ÷ 6 = 7 more days. Total 6 + 7 = 13 days. 12 days assumes Vikas simply replaces Ramesh at the same rate, keeping the pair at 7/day for the whole 84 units. 7 days is only the closing stretch, with the first six days forgotten. 27 days has Vikas finishing the leftover alone $(42÷2=21)$, overlooking that Suresh stays on the job.