RRB NTPC - Clocks and Time

6.

A clock is set right at 5am the clock loses 16min in 24 hours. What will be the right time when the clock indicates 10am? On the 4th day? 

   A.) 10.30pm
   B.) 11pm
   C.) 10.45pm
   D.) 11.15pm

Answer: Option 'B'

Time from 5am. On a day to 10pm. on 4th day = 89 hours. 
Now 23 hrs 44 min. of this clock = 24 hours of correct clock 
365/15 hrs of this clock = 24 hours of correct clock 
89 hrs of this clock = ( 24 × 15/356 × 89) hrs of correct clock
= 90 hrs of correct clock. 
So, the correct time is 11pm. 

7.

At what time between 7 & 8 o'clock will the hands of a watch be in the straight line but no together? 

   A.) 5 3/11 min past 7
   B.) 5 5/11 min past 7
   C.) 5 3/11 min past 7
   D.) 43min past 7

Answer: Option 'B'

When the hands of the clock are in the same straight line but not 
togerther, they are 30 minute spaces apart.
At 7 o'clock, they are 25min. spaces apart. 
Minute hand will have to gain only 5 min. 
space 55 min. spaces are gained in 60min.
5min. spaces are gained in ( 60/55 × 5) min = 5 5/11 min. 
Required time = 5 5/11 min. past 7.

8.

How many times in a day the hands of a clock are straight?

   A.) 22
   B.) 21
   C.) 43
   D.) 44

Answer: Option 'D'

In 12 hours, the hands coincide or are in oppesite direction 22 times. 
In 24 hours, they are at right angles 22 times.

9.

How many times do the hands of a clock coincide in a day? 

   A.) 20
   B.) 21
   C.) 22
   D.) 23

Answer: Option 'C'

The hands of a clock coincide 11 times in every 12 hours 
(Since between 11 and 1, they coincide only once, 
i.e. at 12 o'clock). 
==> The hands coincide 22 times in a day.

10.

How much does a watch lose per day, if its hands coincide every 64 minutes? 

   A.) 32 8/11min
   B.) 36 5/11min
   C.) 90min
   D.) 96min

Answer: Option 'A'

55 min. spaces are covered in 60 min.
60 min spaces are covered in (60/55 × 60)min = 65 5/11 min
Loss in 64 min = ( 65 5/11- 64 ) = 16/11 min
Loss in 24 hrs = ( 16/11 × 1/64 × 24 × 60 ) min
= 32 8/11 min

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