Fajas

*Jan can mow her lawn in 60 minutes by herself, if she hires Wally to do it, he takes 30 minutes, while Peter can do it in 40 minutes. If Jan starts mowing her lawn for 15 minutes then decides to hire Wally and Peter to help her finish, how long will it take (in hours) from when Jan started to when the three finished together??

A 1/6 hours

B 5/12 hours

C 7/12 hours

D 3/4 hours

E 5/6 hours

I just don't know how to do it ! help
thanks in advance

mathwiz90

It takes her 15 min to do her p art (1/4)
then takes 10 minutes for the rest to be done
so i got B

alpha2018

can you explain how you got that answer mathwiz?

Fajas

yea can you please explain how u got that ?

DUNKIN_DONUTS

Together, they can mow 3/40(1/60+1/30+1/30) of the lawn in one minute.
Then you can say that 3/40x=3/4. x=10. 3/4 is the remaining part of the lawn to be mowed(1-(15/60)). 15/60 + 10/60 = 5/12

adam_dewitt7

Dunkin_Donuts is right, but to avoid confusion, I'm just pointing out that he or she meant "3/40(1/60 + 1/40 + 1/30) in that first line. Otherwise he's exactly right.

DUNKIN_DONUTS

oh yeah sorry, that's exactly what i meant. thanks for pointing that out adam.

Fajas

sorry but i still don't get it

awelex

I did it differently (I think), although I used Dunkin_Donuts' explanation as a start. I'll try to explain:

Jan mowes the lawn in 60 minutes. That means that she finishes 1/60 of the lawn in one minute. Wally and Peter can mow 1/30 and 1/40 per minute, respectively. Therefore, the three can mow 1/60 + 1/30 + 1/40 = 9/120 of the lawn in one minute.
They have to mow 90/120 ( = 3/4 ) of the lawn, because Jan has already mowed 1/4. If it takes them 1 minute to mow 9/120, it takes 10 minutes to mow 90/120, because time is proportional to the amount of mowed lawn:

1min --> x = 10 * 1min = 10min
9/120 --> 90/120

Don't forget to add the 15 minutes that Jan mowed the lawn alone:

15min + 10min = 25min = 5/12

Fajas

ahha i finally got it thanx Awelex
thanx for helping everybody !