<PRE><B>Question 125307</B>
Ingrid can rake our lawn in 90 min and I can do it in 60 min. If she rakes for 15 min before I join her, how long will it take us to finish?
:
Let x = time (in min) for you to finish the job
:
Let the completed job = 1
:
{{{15/90}}} + {{{x/90}}} + {{{x/60}}} = 1
:
Multiply equation by 180 to get rid of the denominators:
180*{{{15/90}}} + 180*{{{x/90}}} + 180*{{{x/60}}} = 180(1)
Cancel the denominators and you have:
:
2(15) + 2x + 3x = 180
:
30 + 2x + 3x = 180
:
2x + 3x = 180 - 30
:
5x = 150
x = {{{150/3}}}
x = 30 min</PRE>