Time Equal. Uniform Acceleration. Distance = ???

A body starts from rest and is uniformly accelerated for a minute. Distance traveled in first 20 seconds is x 1 , { x }_{ 1 }, in the next 20 seconds is x 2 , { x }_{ 2 }, and in the last 20 seconds is x 3 . { x }_{ 3 }.

If x 1 , { x }_{ 1 }, x 2 { x }_{ 2 } and x 3 { x }_{ 3 } are in the ratio A : B : C , A:B:C, where A , B , A,B, and C C are coprime positive integers, then what is A + B + C ? A+B+C?


The answer is 9.

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3 solutions

Saqib M
Jul 29, 2014

The initial speed of body will be equal to zero. So, in t 1 { t }_{ 1 } seconds, x 1 { x }_{ 1 } distance is traveled, in t 2 { t }_{ 2 } seconds, x 2 { x }_{ 2 } distance is traveled and in t 3 { t }_{ 3 } seconds, x 3 { x }_{ 3 } distance is traveled. Since s = u t + 1 2 a t 2 s=ut+\frac { 1 }{ 2 } a{ t }^{ 2 } , and when u=0 so in this case it reduces to s = 1 2 a t 2 s=\frac { 1 }{ 2 } a{ t }^{ 2 }

x 1 = 1 2 a t 1 2 { x }_{ 1 } = \frac { 1 }{ 2 } a{ { t }_{ 1 } }^{ 2 } \Rightarrow x 1 = 1 2 a 20 2 { x }_{ 1 } = \frac { 1 }{ 2 } a{ 20 }^{ 2 } = 200a

x 2 = 1 2 a ( t 2 2 t 1 2 ) { x }_{ 2 }=\frac { 1 }{ 2 } a({ { t }_{ 2 } }^{ 2 }-{ { t }_{ 1 } }^{ 2 }) \Rightarrow x 2 = 1 2 a ( 40 2 20 2 ) { x }_{ 2 }=\frac { 1 }{ 2 } a({ 40 }^{ 2 }-{ 20 }^{ 2 }) = 600a

x 3 = 1 2 a ( t 3 2 t 2 2 ) { x }_{ 3 }=\frac { 1 }{ 2 } a({ { t }_{ 3 } }^{ 2 }-{ { t }_{ 2 } }^{ 2 }) \Rightarrow x 3 = 1 2 a ( 60 2 40 2 ) { x }_{ 3 }=\frac { 1 }{ 2 } a({ 60 }^{ 2 }-{ 40 }^{ 2 }) = 1000a

So, * A : B : C* :: 1 : 3 : 5

A + B + C = 9 \boxed{A+B+C = 9}

Nice solution. Slightly quicker than mine. I calculated the velocity at t2 and t3 and added that distance in to an extra 20 seconds of acceleration.

Dorian Thiessen - 6 years, 10 months ago

nice one...

jagdish patra - 6 years, 10 months ago
Jitesh Mittal
Jul 30, 2014

As we know x is directly proprtional to the square of time taken. So distance in first part =x, distance in first part+second part=4x distance in first part+second part+third part=9x We directly conclude the ratio as 1:3:5 and answer will be 1+3+5=9

They are three distances then the answer is 3^2 = 9

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