subtracting time

[Pra4ash]

hi all! I'm confused on how to deduct time.

q
5 hours 14 minutes before 3.00 p.m. (in 24 hrs) ?

My attempt
I got , (3 - 5 = -2 hrs) , (00 - 14 = -14 min)
so, (-2 + 1 = -1 hrs) , (-14 + 60 = 46 min)
answer = -0146 (which totally does make sense!)

Actual answer
0946

where did I go wrong?

 

[MarkFL]

3 pm is 1500, and 5 hours 14 minutes is 6 hours - 46 minutes, hence:

1500 - (600 - 46) = 0900 + 46 = 0946

 

[HallsofIvy]

hi all! I'm confused on how to deduct time.

q
5 hours 14 minutes before 3.00 p.m. (in 24 hrs) ?

My attempt
I got , (3 - 5 = -2 hrs) , (00 - 14 = -14 min)
so, (-2 + 1 = -1 hrs) , (-14 + 60 = 46 min)

Here you added "+1" hour and added 60 minutes. Since you only added the 60 minutes to convert "-14" minutes to "+46" minutes you wanted to add and subtract it:
(-2- 1= -3 hrs). (-14+ 60= 46 min)

answer = -0146 (which totally does make sense!)

and here, after converting "-14 min" to "+46 min" you have treated the minutes as if they were negative, like the hours. Instead -3 hours is 12:00- 3:00= 9:00 AM and then add the 46 minutes: 9:46 A.M

Actual answer
0946

where did I go wrong?

 

[Pra4ash]

You always late for classes, Prash ?

I'll try to make it early

 

[Pra4ash]

thanks all!

 

[guitarguy]

When converting time you can always convert a time like 3:00 PM to 1500 hours and then to 1400 hours and 60 minutes. So subtract 5 from 14 for 9 and 14 from 60 for 46 giving 0946.

This method is the same as borrowing from a column to the left in subtraction with regular numbers in base 10. But the bases are different so you borrow 60 minutes for one hour.