Akansha wrote:How long did it take to travel 400km?
a. The car travelled the first 200km in 2.5hrs
b. If the car's average speed had been 20 km/h faster, it would have travelled
the 400km in 1 hour less time.
Hi!
This question is a perfect place for our data sufficiency hero, "number of equations vs number of unknowns Man", to make an appearance.
Starting by analyzing the stem:
we know that distance = rate * time. We have the distance and we want to solve for the time. Accordingly, we have 1 equation and 2 unknowns; 1 more distinct linear equation and we can solve.
To the statements:
(1) the first 200km took 2.5 hours. No info about the second half of the trip: insufficient.
(2) if the rate had been 20km more, the trip would have taken 1 hour less.
We can convert that info into the following equation:
400 = (r+20)(t-1)
Since we also know that:
400 = rt
we now have 2 distinct equations for our 2 unknowns. However, since our equation has an "rt" term, when we combine them we'll get a quadratic, something that would lead to two solutions if this were pure algebra.
Fortunately for us, this isn't pure algebra: we're talking about rates and speeds, which must be non-negative. So, we can safely discard the negative solution and know that there will only be one possible value for t. Accordingly, (2) is sufficient alone: choose (B).
The "number of equations/number of unknowns" rule is quite possibly THE most powerful weapon in your DS arsenal - be on the lookout for opportunities to use it to wreak destruction on the test!
, If my post helped let me know by clicking the Thanks button 
.
