24 second shot clock

As I see the requirements you need:

1. A two digit display
2. A preset button
3. An alarm when the count reaches zero

Almost nobody builds things out of MSI and SSI logic any longer. Of all the hardware designers out there most of them are D-FlipFlop designers. Less than 5% were ever JK-FlipFlop designers. This is a perfect application for a PIC.

yes, why the requirement for JK FFs?? I'd ask the instructor to relax that requirement. seems silly, I mean why should you have to figure out a counter works?