I made this clock for my sons' Math teacher a couple years ago for Teacher Appreciation.
I did NOT come up with the idea. It was something I found on the internet.
But it was super easy to make. In fact I made another for my husbands office at work. He's an Engineer.
Yes, you could use chalkboard paint, but that's expensive. And chalk wipes off. This is more permanent. The oil pastel ended up working best because is has the LOOK of chalk, but doesn't wipe off. So this cost me a dollar as I already had all the paint and the oil pastel.
And yes...the problems all work out. At least that's what my husband says. I wouldn't really know. :)
Oh, and the math teacher liked it a lot. I'm sure if he were an organized person who cared even a little bit about decorating his classroom, it would be hanging up on his wall. As it is...I think I saw it laying on top of his filing cabinet last year among some piles of papers and books...
On the other hand, it's been a hit at my husband's office. He's had a few other engineers working out the problems to see if they are correct.