Question

The cost of painting a wall that is 5 meters wide and 2 1/2 meters tall is $50. How many square meters can be painted for $1?

Answer 1

Answer: If the a and b of your formula mean how many hours it takes each

person, like a = 2 and b = 1, then the formula gives 3/2 or 1 hour and

30 minutes.  This is not reasonable, since one of the painters alone

could do it in 1 hour.  

Actually, your formula is UPSIDE DOWN. It should be (a*b)/(a+b).

Here's why.  

Let h be the number of hours it takes for the two of them to do it

together. The first person could do it all in a hours, but can paint

only a fraction of the wall working h hours. Assuming a constant rate

of painting, that fraction is h/a. That is, the first painter will

paint h/a of the wall in h hours.  

Similarly, the second painter will paint h/b of the wall in that time.

So, when will they be done? When the whole wall is painted, which

happens whenever those 2 fractions add up to 1.  

You need to solve the following equation:

 (h/a) + (h/b) = 1  

     h*b + h*a = a*b           Multiplying through by a*b

       h*(b+a) = a*b           Factoring out h

             h = (a*b)/(a+b)   Dividing both sides by b+a

Okay, now say 3 painters could do it in a hours, b hours or c hours,

respectively. Let h be the number of hours they all must work to get

it all painted. The first painter finishes h/a of the wall in that

amount of time. The other 2 painters manage to paint h/b of the wall

and h/c of the wall in that time.  Since h is the time to finish the

job, all 3 of these fractions must add up to the whole wall, or 1.  

So you have the equation:

   

       h     h     h        

      --- + --- + ---  =  1

       a     b     c    

 

Solve this for h (start by multiplying through by a*b*c) and you will

get the generalized formula for 3 painters. It's not the formula you

wrote above but I think you can finish it off.

 

(I hope this helps. I'm sorry if it's wrong)