Question

A thief runs with a uniform speed of 100m/minute . After one minute a policeman runs after the thief to catch him. He goes with a speed of 100m/minute in the first minute and increases his speed by 10m/min every succeeding minute. After how many minutes the police man will catch the thief ?

Answer 1

Let the police catch the thief in ‘n’ minutes
Since the thief ran 1 min before the police start running.
Time taken by the thief before he was caught = (n + 1) min
Distance travelled by the thief in (n+1) min = 100(n+1)m
Given speed of policeman increased by 10m per minute.
Speed of police in the 1st min = 100m/min
Speed of police in the 2nd min = 110m/min
Speed of police in the 3rd min = 120m/min
Hence 100, 110, 120… are in AP
Total distance travelled by the police in n minutes =(n/2)[2a+(n – 1)d]
= (n/2)[2 x 100 +(n – 1)10]
After the thief was caught by the police,
Distance traveled by the thief = distance travelled by the police
100(n+1)= (n/2)[2 x 100 +(n-1)10]
200(n + 1) = n[200 + 10n – 10]
200n + 200= 200n + n(10n – 10 )
200 = n(n – 1)10
n(n – 1) – 20 = 0
n2 – n– 20 = 0
n2 – 5n + 4n – 20 = 0
n(n – 5) + 4(n – 5) = 0
(n – 5) (n+4) = 0
(n – 5) = 0 or (n + 4) = 0
n= 5 or n= -4
Hence n= 5 since n cannot be negative
Therefore the time taken by the policeman to catch the thief = 5minutes
Hope it helps you......

Answer 2

Step-by-step explanation:

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here....

theif's speed=100m./min

let time taken by police before catch the thief =x

then the time of theif being catched=x+1

as the police increase his speed=10m./min every minute

his time taken as AP-100,110,120....

Distance of the thief = $time ×speed \\</p><p> =100×(x+1)\\</p><p> =100x+100..........(1)$

Distance covered by police =sum of x terms of AP

=Sn=x/2(2×100+(x-1)10)

=Sn=x(100+5x-5)

=Sn=95x-5x^2...........(2)

as the distance covered by police and theif are same...

=equation 1=equation 2

= 95x-5x^2=100x+100

=5x^2-5x-100=0

= x^2-x-20=0

we solve the equation and get...

x=-4 and x=5

[as the time can't be negative]

= X=5

the time taken by police to catch the thief = 5 minutes