diff --git a/year4/semester2/CT420/materials/prog b/year4/semester2/CT420/materials/prog
index 98e77dba..3401d49f 100644
--- a/year4/semester2/CT420/materials/prog
+++ b/year4/semester2/CT420/materials/prog
@@ -9,7 +9,7 @@
 07 - [X] 03b. CT420 Cyclic Executive Scheduling.pdf
 08 - [X] 03c. CT420 WCET Analysis.pdf
 
-09 - [-] 04a. CT420 POSIX Introduction.pdf
+09 - [X] 04a. CT420 POSIX Introduction.pdf
 10 - [ ] 04b. CT420 POSIX Signals.pdf
 11 - [ ] 04c. CT420 Process Synchronization and RTS.pdf