Normally I don't post questions which are trying to get advice on how to directly a question in a textbook but in this case I feel it is necessary.
The last questions said to write a C program and then set it up as a cron job. I've done that.
The next question asks (so I assume it's possible):
Explain how you can find the time it took the computer to execute your program.
Half the time I'm not sure if the program even executed at all so I'm glad to hear there may be a method to not only check that but how long it took to execute.
How can I find the time taken to execute the program?