83. 
At what time between 7 and 8 o'clock will the hands of a clock be in the same straight line but, not together? 


C
In one hour,once minute hand and hour hand comes in straight line
so our answer is something 7 past,not 8 past
use formula,
titha(0)=30h11/2m
titha is 180,hour hand is on 7
put values
we get titha=60/11
convert into quotitent*remainder/divisor format
we get
5*5/11So answer is 5*5/11 past 7.
82. 
A watch which gains 5 seconds in 3 minutes was set right at 7 a.m. In the afternoon of the same day, when the watch indicated quarter past 4 o'clock, the true time is: 


Error Clock :: Original Clock
3 min 5 sec :: 3 min
185 sec :: 180 sec
[9 hr 15 min = 555 min = 33300 sec]
33300 sec :: (180/185 x 33300) sec
33300 sec :: 32400 sec
333000sec = 9 hrs 15 min :: 32400 sec = 9 hrs [:. 32400/3600 = 9]
So,
Time passed by Error clock = 9 hrs 15 min.
Time passed by Original clock = 9 hrs.Therefore, 9 hrs after 7 am is 4 pm.
81. 
A clock is started at noon. By 10 minutes past 5, the hour hand has turned through: 


Initially both hands are at 12(noon)
1 hr= 30 dgrs ( 12 to 5 = 5 hrs)
so 5*30 = 150 dgrs and
in 60 mins cover 30 dgrs
in 10 mins= 30/60
in 1 min= 30/60*10= 5 dgrs
so
150 + 5 = 155 dgrs
80. 
An accurate clock shows 8 o'clock in the morning. Through how may degrees will the hour hand rotate when the clock shows 2 o'clock in the afternoon? 


Clock is consider as circle 360 deg
the duration of Time= 6 hrs
total = 12 hrs
calculated deg=time duration*deg/total hrs
=6(360/12)=180
79. 
A tank is filled in 5 hours by three pipes A, B and C. The pipe C is twice as fast as B and B is twice as fast as A. How much time will pipe A alone take to fill the tank? 


Let A fill tank in x hrs.
Therefore tank filled in one hrs is 1/x.
And B pipe is twice than A means 2 (1/x).
C pipe is twice than B means 2 (2 (1/x) ) i.e 4/x.
1/x+2/x+4/x = 1/5.
7/x = 1/5.
x = 35 hrs.