4GMAT - Online GMAT Preparation

What should be the approach for these type of questions?

A and B walk up an escalator which moves up with a constant sped. For every two steps that A takes B takes one step. A gets to the top of the escalator after having taken 30 steps while B takes only 20 steps to reach the top. If the escalator were turned off, how many steps should each of them take to reach the top?

1)50
2)80
3)45
4)40
5)60

For every two steps that A takes B takes one step.
Let Speed of A =2 steps per x seconds=2\x;
Let Speed of B =1 steps per x seconds=1\x;
Let Speed of Escalator = N steps per x seconds=N\x;

Time taken for A to climb is (Steps \Time) =30\(2\x)= 15x
Steps actually climbed = 30(by A) +15N----15x(N\x)((Escalator movement for 15x seconds ))

Time taken for B to climb is (Steps \Time) = 20\(1\x)= 20x
Steps actually climbed = 20(by B) +20N----20x(N\x)((Escalator movement for 20x seconds ))

But total steps to be climbed when escalaor is off is the same = 30+15N=20+20N
Hence N=2;
Total steps =60