what is answer to the following:  5 years, 21 days, 4 hours,32
minutes, 17seconds - 2 years, 93days, 7 hours, 47 minutes, 24 seconds.
Second question
How many of the 28 students in a class are boys if 4/7 are girls?

Hello

Let's rewrite 
5 years, 21 days, 4 hours,32 minutes, 17seconds
as
4 years, 385 days, 27 hours, 91 minutes, 77 seconds
-
2 years, 93days, 7 hours, 47 minutes, 24 seconds
So the answer is
2 years, 292 days, 20 hours, 44 minutes, 53 seconds

and 3/7 times 28 = 12 boys.

Happy to help
Richard-ga

Great answer. Appreciate not only the answer but having the info as to
how you arrived at the answer.  My grandaughter thinks I am a genius. 
Thank You