Clocks & Calendar  Vikas Saini

Calender
How to find the day of any century
In a non leap year 365 days.
365 = 7 x 52 + 1.
It means one odd day in a non leap year.
In a year every single day of week repeats by 52 times at least, whereas 1 odd day will be remain in a year itself.
For a leap year 2 odd days will be remain.
In a century we find 76 non leap years and 24 leap years.
So total no of odd days in a century = 76 x 1 + 24 x 2 = 124 days.
124 mod 7 = 5 days.
In a century there is 5 odd days.
100 years = 5 odd days.
200 years = 5 x 2 mod 7 = 3 odd days
300 years = 5 x 3 mod 7 = 1odd day.
400 years = (5 x 4 +1) mod 7 = 0 odd.
It means after 400 years no odd days.
For month odd days code
January  0
February  3
March  3
April  6
May  1
June  4
July  6
August  2
September  5
October  0
November  3
December  5
And code for century
100 years = +4.
200 years = +2.
300 years = 0.
400 years = 1.
An awesome formula to find the day if you know the date :
Day = ( Date + Month code + year + [year/4]+ century code ) mod 7
Q. what was the day on 15th Aug 1947 ?
Date = 15
Month code = 2
Year = 47
century code = 0(1900 = +300)
= (15 + 2 + 47 + [47/4] + 0 ) mod 7
= (15 + 2 + 47 + 11) mod 7
= 75 mod 7
= 5.
5th day of week is Friday.
Q. What was the day 26th January 1950.
Day = 26
Month code = 0
Year = 50
century code = 0.
Day = (26+0+50+[50/4]+0) mod 7
= (26+50+12+0) mod 7
= 88 mod 7
= 4.
4th day of week = Thursday.
Q. What will be the day on 30th September 2016.
Date = 30.
Month code = 5.
Year = 16.
Century code = 1 ( +400)
Day = (30 + 5 + 16 + [16/4] 1) mod 7
= (30+5+16+41) mod 7
= 5
5th day of week is Friday.
Q. What was the day on 12 December 1729.
Date = 12.
Month code = 5
Year = 29.
Century code = +4 (+100 year)
Day = (12+5+29+[29/4]+4) mod 7
= (12+5+29+7+4) mod 7
= 57 mod 7
= 1
1st day of week is Monday.
Clocks
Finding angle between Hands
12 h = 360°
1 h = 30°
60 m = 30°
1 m = 0.5°
Angle =  5.5 M  30 H 
M = minute
H = Hour
Q. What is the angle between 4:45 PM ?
solution :
5 hours = 5 x 30° = 150°
45 min = 45 x 0.5° = 22.5°
Total = 150°  22.5° = 127.5°
using Shortcut :
Angle =  5.5 M  30 H 
=  5.5 x 45  30 x 4 
= 127.5 °
Q. What is the angle between at 3:30
solution :
3 Hours = 30° x 3 = 90°
30 min = 0.5° x 30 = 15°
Total = 90°  15° = 75°.
Shortcut :
Angle =  5.5 M  30 H 
=  5.5 x 30  30 x 3 
= 75°
Relative speed of 2 clock hands
12 H = 60 div
1 H = 5 div.
SH = 60 M = 50 div.
SH = 5/60 div/min.
SH = 1/12 div/min.
SM = 60 div / 60 min = 1div/1min
Relative speed = 1  1/12 = 11/12 div/min.
T = 60 / (11/12)
T = 65 + (5/11)
T = 1:05:5/11
Both hand will coincide after every 1:05:5/11.
Q. By what time both hands will coincide in between 56 PM ?
solution :
(1:05:5/11) x 5
= 5:25:25/11
= 5:27:3/11
Q. By what time both hand will coincide in between 9  10 PM ?
solution :
(1:05:5/11) x 9
= 9:45:45/11
= 9:49:1/11.
Q. By what time in between 23 PM both hand will make angle of 30°.
solution :
angle =  5.5 M  30 H 
30° =  5.5 x M  30 x 2 
30° =  5.5 M  60° 
90° = 5.5 M
M = 900 / 55 = 180/11 = 16 (4/11).
Time = 2:16:4/11.
Q. By what time in between 56 PM both hands will make angle of 60°.
solution :
angle =  5.5 M  30 H 
60° =  5.5 M  30 x 5 
60° = 5.5 M  150
210° = 5.5 M
M = 2100/55 = 420/11 = 38(2/11)
Time = 5:38:2/11.
Q. A watch gains 5 sec in every 3 minutes and was set right at 8 AM,so what time will it show at 10 PM on the same day ?
solution :
3 Min = 5 sec
1 min = 5/3 sec.
60 min = 100 sec.
1 Hour = 100 sec.
14 Hour = 1400 sec.
1400 sec = 23 min 20 seconds.
Time = 10:23:20 sec.