Yes D is the answer.

How do you feel Knewton's method of teaching? I am planning to give it a try. Are you learning anything new? Do you feel it is useful.

THanks in advance.

Given to us : Distance is 2^m+1

speed of Tm = 2^m
speed of Tn = 2^n
n > m
speed of Tn relative to Tm = 2^n - 2^m = 2^m {( 2^(n-m)) -1}

S = d / t

time = 2^m*2 / 2^m {( 2^(n-m)) -1}
time = 2/2^(n-m) -1

Let me know if the solution looks incorrect.

newton9,

I took knewton course early this year. To be honest I did not find the course very helpful on verbal side. The course goes over most of the general topics in verbal and most of the problems are similar to OG or based on OG type questions. If you are looking for something that can build your concepts than I would not suggest the course. If you have already done most of the prep work and you are looking for something to get you in the grove then go for it.

Before you make any decision do try their free class just to get an idea how the course works.

hey Bhumika,
the easy way to solve such problems is to plug in numbers.
i assume, m= 2 and n=3
accordingly, speed of train m= 4 miles/hr
speed of train n = 8 miles/hr

the distance between them is 8 miles.

according to relative velocity, when two objects are moving in the same direction then their speeds subtract (8-4 = 4)

to find the time:distance/speed
8/4 => 2 hrs.

now substitute the values of m and n in the answer choices and see which option gives you the value of 2.
Only D does that.

Hope this helps!

bhumika.k.shah wrote:This is the best i could get to put this q here.

Source - knewton CAT

Bhumika,

This one deals with Relative speeds.
Since n > m, relative speed between the 2 trains is ((2^n)-(2^m)){ read( 2 power n) -(2 power m))

then , Time taken to catch up =Distance /Speed
So apply the values and U can easily see the option D to be poping up there...

Sometimes forumal helps..

I am not against plugging numbers, but TIME is our enemy!!