It takes a boy 40 minutes to mow the lawn, but his sister can mow it in 60 minutes. How long would it take them to mow the lawn if they worked together, using two lawn mowers?

Question

anonymous · Answer

24 minutes.. is the answer correct?

hero · Answer

http://openstudy.com/code-of-conduct

anonymous · Answer

Sorry..

boy will mow 1/40 lawn in 1 min..
girl will mow 1/60 lawn in 1 min..
so togther, 1/40 + 1/60  = 1/24 lawn would be mowed in 1 min.

So 24 minutes...

anonymous · Answer

it is thanks! How did you go about doing it? I'm confused

anonymous · Answer

Perfect Thanks!!

hero · Answer

Alternative method:

B = Time it takes brother to mow
S = Time it takes sister to mow
t = time it takes both:

$$\large\frac{B \times S}{B + S} = t$$