GMAT Prep2 ?? (Kilometers)

dferm wrote:Please Help..

Thanks....

So we know Rate x Time= Distance. In the stem we're given the distance (400km). So we have one linear equation: RT= 400 and we're asked for T. We can solve if we solve for R or another unique linear equation for R & T.

1. 1st 200km took 2.5 hours. Doesn't give us much. Insufficient.

2. If R were 20km greater then the car would have traveled 400km in 1 hour faster. Algebraically: (r+20)(t-1)= 400. This is a unique and linear equation. You don't have to bother to do the computation to get an answer, but you could and that's the point. Sufficient.

It's B.

Let speed = R, time = T. From question, 400 = RT ---- equation (1);

(1) Insufficient, no info about the other 200 miles

(2) Sufficient, we can write a second equation ---- 400 = (R+20)(T-1) --- eqaution (2). If you substitute equation (1) and (2), we can solve.