Question

A new crew of painters can paint a small apartment in 12 hours. AN EXPERIENCED crew can paint the small apartment in 6 hours. How many hours does it take to paont the apartment when the two crews work together?

Answer 1

if both crews work together, let's say the finish the job in "t" hours.

so.. in 1 hour, they have done 1/t of the whole work.

now, the new crew, working by itself can do the whole job in 12 hours, that means, in 1 hour, they have done only 1/12 of all the work.

the experienced crew, can do the job in 6 hours, that means in 1 hour, they have done 1/6 of all the work.

now, let's add their rates for 1 hour worth, to see what we get.


`\bf \begin{array}{clclcllll} \cfrac{1}{12}&+&\cfrac{1}{6}&=&\cfrac{1}{t}\\ \uparrow &&\uparrow &&\uparrow &&\\ new\ crew&&experienced&&total\\ rate/hr&&rate/hr&&work/hr \end{array}\\\\ -------------------------------\\\\ \textit{let's multiply both sides by \underline{12t}, to toss away the denominators} \\\\\\ t+2t=12`

and pretty sure you know how much that is.