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

[charmaine]

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

thankssss

[VerbalAttack]

Hi Charmine,

assume the following;
tp = Time taken for Car P to travel 600miles
rp = Rate of Car P

tr = Time taken for Car R
rr = Rate of Car R

As per the question; tp = tr - 2 & rp = rr + 10


Use formula Rate = Distance / Time;

tp = 600 / rp and tr = 600 / rr

tp = 600 / rp ==> tr - 2 = 600 / (rr + 10) ==> replace tr with 600 / rr

==> after simplifying, we get ((6000/rr) - 2rr) = 20.

From here backsolve with answer choices.

You can also continue to solve the equation, which gives rr = -60 & rr = 50. As Rate can't be negative, take rr = 50.


Cheers

[charmaine]

thank you ! much appreciated

[egybs]

i did the same thing as verbalattack, but it seems messy. I think this is a perfect opportunity to just throw in numbers and not bother solving for p or r.

[netigen]

Back solving is idle here with a 30 sec soln but we can also try this the Maths way:

Tp = Tr - 2

600/Sp = (600/Sr) - 2

we know that Sp = 10 + Sr

600/(10+Sr) = (600/Sr) - 2

600/(10+Sr) = (600 - 2Sr)/Sr

600Sr = 6000+600Sr - 20Sr - 2(Sr^2)

2(Sr^2) + 20Sr - 6000 = 0

Sr^2 + 10Sr - 3000 = 0

Sr = -60 or Sr =50

since speed can not be -ve so Sr=50

[Scott@TargetTestPrep]

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