Sinclair ZX Spectrum. I bought it for around 2000 SEK on my 11th birthday and learnt a lot about how computers work, both in hardware and software.

Here's a problem I've seen asked several times:
Calculate the number of office hours between two given times. Office hours are Monday - Friday 8.00 - 18.00.

How would you solve this in an efficient way? Actually, knowledge of integration might be useful.

Maybe it's the cows that need to be debugged, not the software.