Aptitude - Time and Distance - Discussion

Saravanakgopi can you please tell me that how you get 5400 instead of 600 ?

I don't why but this is the second time they have used the difficult method. They use the basic principle to assume the variable to find as x i.e. time of flight as x. I don't follow this always.

Let's do the opposite way. Let us assume the original (or misleadingly called average) speed as x. Now if speed is decreased.

Then the time is increased by 30 mins i.e. 30/60 hrs (since we are using kmph so time has to be in hrs).

Now using common sense logic. If less speed then more time to travel the same distance.

So here the time difference for the two speeds is 30 mins or 30/60 hrs.

i.e (Time with less speed) - (Time with faster speed) = 30/60 hrs.

Now Time = Distance/Speed.

(600/(x-200))-600/x = 30/60.

Solve this equation to get x = 600 kmph.

600 km/600 kmph = 1 hr simple as that.

Just seems lengthy because I explained the logic. But its not always good to assume the unknown as x. This method seems easier to me.

Let: Time = T, Speed = x

600/x = t ----> (1)
600/x - 200 = T + 1/2,
600/x - 200 = 3/2 T,
600 * 2/(x - 200) * 3 = T.
1200/3x - 600 = T---> (2)

BySolving..

1200/3x - 600 = 600/x.
3x - 600 = 2x.
x = 600.

Hence Speed is 600.

Since D/S = T.
D = 600.
S = 600.
T = 1.

Thanks for your solution @Kiran.

S inversely proportional to time;

Time increases flight speed decreases
T1s1=T2S2
X*600=(x+30)400
6x=4x+120
X=60.

To solve this problem within a second means. First find the equation.
Speed is reduced by 200kmph.
So,
Speed1 - speed2. = 200kmph.
D/t1 - D/(t2+ 1/2) = 200kmph.
600/x - 600(x+1/2) = 200kmph.
Substitute all the options given in the question one by one.
You will LHS =RHS.

How do we get x(2x + 1) = 3? please explain.

Thank you @Muzzu.

Good. Thanks @Vijaya Laxmi.

It must be 1.5 hours.