Naomilobo16
Hi Scott, how did you decide on the number 60?
ScottTargetTestPrepScottTargetTestPrep
Bunuel
A train meets with an accident and travels at 4/7th of its regular speed hereafter and hence it reaches its destination 36 minutes late. How much time does the train take to reach its destination from the site of the accident had it traveled at its regular speed?
A. 40 minutes.
B. 42 minutes.
C. 46 minutes.
D. 48 minutes.
E. 50 minutes.
Solution:
We can let 60 mph be 4/7th of the regular speed of the train; thus, the regular speed of the train is 60 / (4/7) = 60 x 7/4 = 105 mph. If we let t = the time, in hours, it takes the train to reach its destination from the site of the accident at its regular speed, we can create the equation:
105t = 60(t + 36/60)
105t = 60t + 36
45t = 36
t = 36/45 = 4/5
We see that it takes the train 4/5 of an hour, or 4/5 x 60 = 48 minutes, to reach its destination from the site of the accident at its regular speed.
Answer: D Explanation:
Hi Naomilobo16, here's an explanation of why I chose 60 mph for the train speed.
When it is possible to solve a question by picking numbers, we want to choose our numbers in such a way that the arithmetic that follows is easy to perform. For instance, working with fractions is generally harder than working with integers, so if the problem involves division, we choose numbers which are multiples of the divisors in the question. If the problem involves taking square or cube roots, we choose numbers which will not produce any radicals (if possible). If the problem involves percents, we choose multiples of 100 so that we get whole numbers when we take percents. We also try to keep our numbers as small as possible so that the calculations are easy.
For this problem, before deciding the number to choose, we scan the problem and determine that the number we choose will be divided by 4/7 at some point. Therefore, the only condition when choosing a value for the (reduced) speed of the train is that the number should be divisible by 4. We chose 60 mph because it is divisible by 4 and it seems a reasonable value for the speed of a train. We could actually use 4 mph and arrive at the same answer; here’s how:
If 4/7 of the regular speed of the train is 4 mph, then the regular speed of the train is 4 / (4/7) = 7 mph. Let t be the number of hours for the train to reach its destination, as in my original solution. Then:
7t = 4(t + 36/60)
7t = 4(t + 3/5)
7t = 4t + 12/5
35t = 20t + 12
15t = 12
t = 12/15 = 4/5
As we can see, we obtained the same value of t = 4/5 by choosing the reduced speed of the train to be 4 mph instead of 60 mph. Any multiple of 4 would work the same way; however, I like to choose values that agree with real life, and one doesn’t see a lot of trains traveling at 7 or 4 miles per hour. That’s why I chose 60 mph.