GMATPrep - 400KM in the wrong lane

[AleksandrM]

[jasonc]

1) insuff. Tells us nothing about speed (could be different in the first vs second 200km)

2) gives us the following equation:
(r1+20) t2 = 400km
(r1)(t1) = 400km
t2-t1 = 1 hour

3 equations, 3 unknowns = suff.

[AleksandrM]

jason,

you've exposed a pretty major flaw in my approach to DS problems. I often focus too much on actually solving the problem, rather than just getting enough information to show me that there is enough info to get the answer.

Sometimes, I have to go through more elaborate steps when dealing with number properties, but for the most part I should really focus on just establishing sufficiency and saving 30 or 40 seconds here and there.

[jasonc]

yes, your time management should improve drastically once you start solving for sufficiency instead of actual numbers.

I use to suffer from the same issue. I recommend going through all of the OG11's DS questions, if you learn to solve them all in under 2 min, you'll learn many tricks to apply to different DS questions to solve for sufficiency.

[airan]

It can be done with 2 equations also.
Let the speed be s and time be t.

400/s=t

400/s+2= = t-1, so 2 equations with 2 variables. can be solved.
Answer is B