Word problem
posted by Jennifer on .
Edward can paint a bedroom in 3hr. Jeb can paint the same room in 3.75 hours. How long will it take them, working together, to paint the bedroom?

Their average time for painting the bedroom is 3.375 hours. Working together they'd take 3.375/2 = 1.67875 hours.

This is actually a multiple choice question and the choices are:
a.)20/3 hr
b.)2/13 hr
c.)5/3 hr
d.)1.5 hr
Would the answer be c? 
How did you know to take the average of 3.75 and divide it by 2?

I must respectfully disagree with Ms. Sue
Edwards rate = room/3
Jeb's rate = room/3.75
their combined rate = room/3 + room/3.75
= (3.75room + 3room)/(3x3.75(
= 6.75room/11.25
so Time = room/(6.75room/11.25)
= 11.25/6.75
= 1.666... hours
I know that my answer is "very close" to Ms. Sue's, but they are not the same.
Had the two times to paint the room been more diverged, the difference in the times would have been more obvious.
This question falls in the same category as the following:
Bob drives 100 miles at 50 mph, then Steve takes over and drives the next 100 miles at 80 mph.
What was the average speed for the trip.
the temptation would be to say 65 mph since they both drove the same distance, but....
time taken by Bob = 100/50 = 2 hours
time taken by Steve = 100/80 = 1.25 hours, for a total time of 3.25 hours
so average rate = 200/3.25 = 61.5 mph 
This is actually a multiple choice question and the choices are:
a.)20/3 hr
b.)2/13 hr
c.)5/3 hr
d.)1.5 hr
Would the answer be c? 
correct
5/3 = 1.6666.... 
Thank you so much. I understand now.