Explain how you know that when finding a fractional part of a whole number is the same as multiplying that fraction by a whole numbver

I know that because my 8th grade math teacher taught us (in 1954)
that when you see the word "of" in a math problem, it almost always
means "times".  So part of your question says "... finding a fractional
part times a whole number ...".
 
 

3were boys and 5 were girls

If she spent  3/8 playing games and 50% eating, that would be 4/8. so she would have had to spend 1/8 opening presents