carcass
Each machine at a toy factory assembles a certain kind of toy at a constant rate of one toy every 3 minutes. If 40 percent of the machines at the factory are to be replaced by new machines that assemble this kind of toy at a constant rate of one toy every 2 minutes, what will be the percent increase in the number of toys assembled in one hour by all the machines at the factory, working at their constant rates?
A. 20 %
B. 25 %
C. 30 %
D. 40 %
E. 50 %
I guess, no one has used the weighted averages method for this question and hence posting this solution here for everyones benefit.
We are asked to determine the percent increase in number of toys over the original number of toys.
Option 1 If all the machines were old, working at 1 toy/3 min... then we will get 20 toys...
If all the machines were new, working at 1 toy/2 min... we will have 30 toys...
The ratio of new machines is 3:2...
Old (3) New (2)
20 -------------------------------------------------------30
We can see that the weighted average is 24... which will be 20% more than the current output.
Option 2We are given the rates of old and new machines, and asked to find the latest rate if the ratio between old and new machine is 3:2
If all the machines were old, working at 1 toy/3 min... then we will get 20 toys...
If all the machines were new, working at 1 toy/2 min... i.e. 1.5 toys in 3 mins.
The ratio of new machines is 3:2...
Old (3) New (2)
1 -------------------------------------------------------1.5
We can see that the weighted average is 1.2... which will be 20% more than the current output.
Even though, I have posted the above method, I don't know whether I would be able to recollect this approach under time pressure during exam... and might have gone ahead with the number plugging approach here (The Good Old Fashioned Way

- Taking 5 machines, and calculating the output of each machine as per old and new rates... and finding out the percent increase)