rate problem !!!!!!!!!!pls help

charmaine wrote: ↑

Mon Jun 09, 2008 1:07 am

i do not know how it comes about ..i used algebra equation as solution bt it seems wrong


the time it took car P to travel 600 miles was 2 hrs less than the time it took car R to travel the same distance. if P's average speed was 10 miles per hour greater than that of car R, what was R's average speed, in miles per hour?

a. 40
b. 50
c. 60
d.70
e. 80
answer is b 50

Solution:

Since the answer choices are easy numbers, let’s solve the problem numerically instead of algebraically. That is, we can check each answer choice until we find the correct answer.

If car R’s speed is 40 mph, then car P’s speed is 50 mph. Furthermore, it will take car R 600/40 = 15 hours and car P 600/50 = 12 hours to travel 600 miles. However, since 12 is not 2 less than 15, we see that choice A is not the correct answer.

If car R’s speed is 50 mph, then car P’s speed is 60 mph. Furthermore, it will take car R 600/50 = 12 hours and car P 600/60 = 10 hours to travel 600 miles. However, since 10 is 2 less than 12, we see that choice B is the correct answer.

Answer: B