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chelle21689's avatar

Did I find the time of each worker correctly in this algebra question?

Asked by chelle21689 (7907points) November 29th, 2010

Ty and Ey are painting a house. Together it will take them 24 hours to paint the house. Ty alone can paint the house in six hours less than Ey working alone. How long to the nearest hour would it take each of them working alone?

X=Ey Y=Ty
x+y=24 y=x-6

x+x-6=24
2x=30
x=15

15+y=24
y=9

x=24+15 so x=39 minutes

y=24+9 so y=33 minutes

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7 Answers

WestRiverrat's avatar

Check your units or measure.

chelle21689's avatar

Haha thanks…I wish I could delete this now.

gasman's avatar

my answer deleted by me

gasman's avatar

Your solution was nicely written with its own internal logic but as you might suspect completely wrong. Remember that the two of them working together finish the house in fewer hours than either alone, so you can’t just add hours. It’s tricky that way. Time is the reciprocal of rate. Here’s what I mean…

Let T = Ty’s rate of building, in houses per hour—presumably a small fraction)
and E = Ey’s rate of building (likewise)

Then the stated problem translates as follows:
1/T + 1/E = 24 (hours per house)
1/T = 1/E – 6 (hours per house)

This leads to a simple solution…
Good luck working it out!

gasman's avatar

Oops, I goofed & my answer was also completely wrong. My second equation is valid, but with the first equation I fell into the same trap you did, adding hours together. The quantity that is additive is the rate of painting in units of houses/hr. T+E represents their combined effort.

The correct first equation is therefore
1 / (T+E) = 24 (hours per house)

I solved for T in terms of E (2nd eq) and substituted into the above to yield a quadratic, with irrational roots, one positive and one negative. The positive number makes perfect sense, however, and satisfies the conditions of the problem.

Turned out to be more difficult than anticipated.

gasman's avatar

OK, a more minor goof this time—made a mistake in deriving the quadratic & the coefficients were off a little. Both roots are positive & irrational but only one makes sense. Without revealing the exact answer, I can tell you that Ty alone takes almost 2 days to paint a house and Ey alone takes over 2 days.

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