Content update and format improvement (#28840)

* Content update and format improvement

Simplified the content, added subheadings, and formatted the examples.

* Update index.md

guide/english/mathematics/adding-and-subtracting-time/index.md

@@ -7,66 +7,70 @@ There are a few basic rules that guide addition and subtraction of time.
 
 Here they are:
 
-60 seconds make 1 minute
-60 minutes make 1 hour
-1440 minutes make 1 day
-24 hours make 1 day
-7 days make 1 week
-52 weeks make 1 year
-365 days make 1 year
-366 days make 1 leap year
-12 calendar months or 13 lunar months make 1 year
+* 60 seconds make 1 minute<br/>
+* 60 minutes make 1 hour<br/>
+* 24 hours make 1 day<br/>
+* 7 days make 1 week<br/>
+* 52 weeks make 1 year<br/>
+* 365 days make 1 year<br/>
+* 366 days make 1 leap year<br/>
+* 12 calendar months or 13 lunar months make 1 year<br/>
+
+### Adding
+Let us see an addition example below by adding Line 1 to Line 2:
+
+       
+       Line 1: 3hr 45min 20sec
+       Line 2: 2hr 20min 50sec
+               --------
+               5hr 65min 70sec  (convert 60 seconds to a minute)
+               --------
+               5hr 66min 10sec  (convert 60 minutes to an hour)
+               --------
+               6hr 06min 10sec  (final answer)
+
+First, we perform the addition for each time period (seconds, minutes, and hours). Then, starting with the smallest time period (seconds), we check if the value can be converted to the next level of time period. This ensures that the final value for each time period stays below the allowable maximum.
+
+### Subtracting
+In the example below, we subtract Line 2 from Line 1:
 
 
-For example, subtracting 1 hour 15 minutes 30 seconds from 4 hours would result in the following
-
-Line 1: 4 00 00
-Line 2: 1 15 30
-        --------
-        2 44 30
-        --------
+        Line 1: 4hr 00min 00sec
+        Line 2: 1hr 15min 30sec
+                --------
+                2hr 44min 30sec
+                --------
 
 
-On Line 1 above, (subtraction and addition) will start from the right most numbers (in this case 00). Deducting 30 from 0 is normally impossible, so what we would do is go the next set of numbers (00) to see if we can borrow. In this case, it's impossible so we go to the next number (leftmost which in this case would be 1 hour- remember 60 minutes make 1 hour).
+For subtraction involving time, a good practice is to start from the smallest time period (the rightmost value in the above example which is seconds). Since we are unable to deduct 30 from 0, we will have to go the next higher level of period (minutes) to borrow a unit. In this case, since the minutes are zero we go to the next period which is hour (remember 60 minutes make 1 hour).
 
-This is what it would like at this point
+Borrowing from the hour period and breaking down the hour into minutes:
 
-3 60 00
-1 15 30
+        3hr 60min 00sec
+        1hr 15min 30sec
 
 
-Next we need to borrow 1 minute from 60, resulting in what we have below. Remember however that 60 seconds make 1 minute. Reducing 60 by 1 unit results in 59, so we have:
+Borrowing a minute and break it down into seconds:
 
-3 59 60
-1 15 30
+        3hr 59min 60sec
+        1hr 15min 30sec
 
 
-By now we can then make our deductions
+The operation can now be performed:
 
-60 minus 30 is 30
-59 minus 15 is 44
-3 minus 1 is 2
+       Seconds: 60 - 30 = 30
+       Minutes: 59 - 15 = 44
+       Hours:   3 - 1 = 2
 
 
-Hence subtracting 1 hour 15 minutes 30 seconds from 4 hours would result in 2 hours 44 minutes 30 seconds.
-
-This playful youtube video for kids might also be useful as it goes down to the very basics.
-https://www.youtube.com/watch?v=HrxZWNu72WI
+Hence subtracting 1 hour 15 minutes and 30 seconds from 4 hours would result in 2 hours 44 minutes and 30 seconds.
 
 
-Let us see an addition example by adding 3 hours 45 minutes 20 seconds to 2 hours 20 minutes 50 seconds
+## More information:
 
-H M  S
-3 45 20
-2 20 50
+[YouTube video](https://www.youtube.com/watch?v=HrxZWNu72WI
+) - intended for kids which cover the basics in a fun way.
 
 
-20 seconds plus 50 seconds is 70 seconds. Recall 60 seconds makes 1 minute. 70 seconds therefore will be 1 minute 10 seconds
-45 minutes plus 20 minutes is 65 minutes. Recall 60 minutes is 1 hour. 65 minutes therefore will be 1 hour 5 minutes
-3 plus 2 is 5 (hours)
-
-we then need to do the additions.
-
-5 hours + 1 hour 5 minutes + 1 minute 10 seconds would be 6 hours, 6 minutes and 10 seconds.