You need to think if your answer makes sense. Suppose you were out sick 364 days. That would be a very high percentage, in the 90's. But 364/365 is less than 1, so (364/365)% is less than 1%! So something is wrong!
The person is out sick 100 days out of 365 days. Out of means divide (and divide means out of!). So 100 days out of 365 days is the same as (100 days / 365 days) = 100/365 as the days cancels out. But you want percentage, so you need to change the way 100/365 looks (you want a % symbol). Hence you multiply by 1 to get something to look differently. There are many ways of writing 1 and one such way is 100%-and it has a % symbol!
So 100/365 = (100/365)* 1 = (100/365)*100% = .274*100% = (.274*100)% = 27.4%