Method 1
16 - 5 =11 days' work remains by the original 60 bricklayers
[60 x 16] / 55 =17.4545.....days that would have taken 55 bricklayers to finish the entire work.
But 5/16 of the work is already done and 11 /16 remains.
So: 17.4545... x 11/16 =12 days for the remaining 55 bricklayers to finish the work
12 + 5 days =17 days, or 1 extra day for the 55 bricklayers to finish the work.
Method 2
Since 60 / 16 =3.75 old rate and 55 / 11 =5 new rate
Therefore: 3.75 / 5 x 16 days =12 days, or 1 extra day for 55 bricklayers to finish the work.
12 + 5 ==17 days, or 1 extra day.
Method 3
55 / 60 = 0.9166667 of the original 60 bricklayers remain on the job. Therefore the number of days to finish the work will go up to: [1 / 0.9166667] x 11 days = 12 days.