Hi shibsriz,

This question CAN be solved with either algebra or TESTing THE ANSWERS. Since you asked about how to solve by using the answers, here's how you would do it....

This question involves the Distance Formula

Distance = Rate x Time

We're told that the distance = 280km, so

280 = R x T

The question also mentions an exact difference of 30 minutes, which is a "round number" (relative to time). It makes me think that the question is probably designed around other round numbers.

While I would normally start with answers B or D when TESTing the Answers, here I'm going to start with 80 (since it's a round number)...

So if X the original speed and X = 80, we'd have.

280 = 80 x T
280/80 = T
T = 3.5 hours

Now let's see what happens when we subtract .5 hours (since the bus left 30 minutes late) and increase the speed by 7/6...

80(7/6) x (3) = ???

560/6 x 3

1680/6 = 280

This MATCHES the original distance, so it MUST be the correct answer.

Final Answer: C

GMAT assassins aren't born, they're made,
Rich

Hi hlobprise,

Tushar14 has properly translated the information into 2 equations, which could then be used to answer the question. I'm going to add a few more details to his explanation.

This prompt is based on the Distance Formula:

Distance = Rate x Time

The original prompt tells us that the distance is 280 kilometers and refers to the rate as X kph. Using this information, the original equation can be written as....

280km = (X kph) x (T)
280 = XT

The next sentence changes the information: the bus leaves 30 minutes (1/2 an hour) late and drives 7/6 the speed. This can be written as....

280km = (7X/6 kph) x (T - 1/2)
280 = (7X/6)(T - 1/2)

With these two equations, you can now solve algebraically.

GMAT assassins aren't born, they're made,
Rich