It takes the high-speed train x hours to travel the z miles from Town ...
A question with variables in the answer choices (VIC) can be solved by picking values for the variables.
Let's pick the following values for x, y and z:
When picking values for a VIC question, it is best to pick numbers that are easy to work with (i.e., 12 is divisible by 4 and 6 here), but that don't have any extraneous relationships between them. For example x = 4, y = 3, z = 12 would be a less favorable set of numbers because xy would equal z in that case and there is no need for the product of the two times to equal the distance. Picking variables with extraneous relationships can lead to false positives when checking the answer choices.
Now let's solve the question according to the values we selected.
If the high-speed train travels the 12 miles from A to B in 4 hours, it is traveling at 3 mph.
If the regular train travels the 12 miles from A to B in 6 hours, it is traveling at 2 mph.
To evaluate how far each train travels when they move toward each other starting at opposite ends, let's set up an RTD chart.
We can set-up two equations with two unknowns and solve
In the 2.4 hours it takes for the two trains to meet,
the high speed train will have traveled 3(2.4) = 7.2 miles,
and the regular train will have traveled 2(2.4) = 4.8 miles.
Therefore the high speed train will have traveled 7.2 – 4.8 = 2.4 miles farther than the regular train.
2.4 is our target number.
Let's see which of the five answer choices give us 2.4 when we plug in our values for x, y and z:
Only A matches the target.
This question can also be solved algebraically.
Since the trains traveled the z miles in x and y hours, their speeds can be represented as z/x and z/y respectively.
We can again use an RTD chart to evaluate how far each train travels when they move toward each other starting at opposite ends. Instead of using another variable d here, let's express the two distances in terms of their respective rates and times.
Since the two distances sum to the total when the two trains meet, we can set up the following equation:
zt/x + zt/y = z - divide both sides of the equation by z
t/x + t/y = 1 - multiply both sides of the equation by xy
ty + tx = xy - factor out a t on the left side
t(x + y) = xy - divide both sides by x + y
To find how much further the high-speed train went in this time:
(
ratehigh × time) – (
ratereg × time)
(
ratehigh –
ratereg) × time
The correct answer is A.