GMATPrep - 400KM in the wrong lane

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.

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.

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.

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