A homeowner estimates that it will take 11 days to roof his house. A professional roofer estimates that he could roof the house in 6 days. How long will it take if the homeowner helps the roofer?

If the homeowner helps the roofer it will take 4 days to roof the house.

Step-by-step explanation:

Given that a homeowner estimates that it will take 11 days to roof his house, while a professional roofer estimates that he could roof the house in 6 days, to determine how long it will take if the homeowner helps the roofer the following calculation must be done:

1/6 + 1/11 = X

0.1666666 + 0.09090909 = X

0.25 = X

1/4 = X

Therefore, if the homeowner helps the roofer it will take 4 days to roof the house.

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4