Reference answer: 26
100. $A, B, C$ are all nurses, starting from January 1, 2019, $A$ works for 3 days and then rests for 1 day, $B$ works for 5 days and then rests for 2 days. If both $A$ and $B$ rest on the same day, then $C$ will substitute for them. How many days did $C$ substitute in 2019? $\qquad$