Mary stopped work at 4:15 this afternoon she worked 6hr and 45min when did she began working ?

Question

I got a weirs answerd

Answer 1

Here is an opportunity to subtract larger numbers from smaller ones by borrowing from the next higher number position.
 
Note: When we subtract in decimal, the next higher position is 10 times this one.  So, for example:
     42
   -  7       (since we can't subtract 7 from 2, so we borrow 1 from the 4,
---------              making it a 3 and this a 12)
     35       (  12-7=5 and we just bring down the 3 that was once a 4 )
 
When subtracting larger minutes from smaller ones, you must borrow an hour (60 minutes) from the hour's position.
 
When subtracting larger hours from smaller hours, you must decide whether to use a 12-hour clock (AM/PM) or a 24-hour clock.  Since this problem used the words, "this afternoon," we know that they are using a 12-hour clock and meant 4:15 p.m.  We can borrow hours back to Noon and still have p.m. (note: is 2 minutes after Noon 12:02 a.m. or 12:02 p.m??), but if we borrow any more, we will have to go back to a.m.  And, if we borrow still more, we will have to go back to yesterday's p.m.
 
So,
   4:15   p.m.
  -6:45 hours
 -------
         0      (ah! now we're stuck, we need more minutes.  So we borrow an hour:

 
    3:75  p.m.    (it looks funny, but it is the same time, just not reduced)
   -6:45  hours
   ------
      :30      (oh, no! stuck again.  We need to go from p.m. back to a.m., so borrow 12 hours because
                    3 p.m. is the same as 15 a.m.  -- again, just not reduced yet)

 
   15:75 a.m.       (this is why people like me use a 24-hour clock! We can't subtract very well.)
   -6:45
  --------
    9:30 a.m.         (cool! glad we didn't have to back to yesterday!)position.

Answer 2

9:30 a.m.

 

If you back up 6 hours from 4:15 p.m. you are at 10:15 a.m.

45 mins prior to that is 9:30 a.m.