Working at their respective constant rates, machine A makes copies in minutes and machine B makes copies in minutes. If these machines work simultaneously at their respective rates for minutes, what is the total number of copies that they will produce?
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Wait, I got 750 but I think I messed up. I did A: 100/12 per min, B: 150/10 per min, then added and multiplied by 30. Got 250 + 450 = 700? No, 100/12*30=250, 150/10*30=450, sum 700. Hmm but I picked 750. Let me recheck.
Yeah, 100/12 is 8.33 per min, times 30 is 250. B is 15 per min, times 30 is 450. Total 700. So D.
Ah I see, I accidentally did B as 150/10=15, but then 15*30=450, added to 250 gives 700. I must have added wrong earlier.
This was pretty straightforward. Just find individual rates per minute and add them, then multiply by total time.
Agreed, sub-505 for a reason. No tricks here.
I like to combine rates first: A+B = 100/12 + 150/10 = 100/12 + 15 = 100/12 + 180/12 = 280/12 = 70/3 per minute. Then times 30 = 700. Saves a step.
Nice, that's cleaner. I did it separately but same result.
What it tests
Your ability to combine individual work rates, usually by working with the fraction of a job completed per unit of time.
Common trap
Adding the times instead of the rates, or forgetting to take the reciprocal when combining rates.