Question 1198709
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Consider a field with 300 plants of potatoes.
There's nothing particularly special about this value. I chose it so that the subsequent division operations result in whole numbers shown below. Feel free to try out other values. The answer will result the same each time.


In this hypothetical numeric example, the goal is to harvest 300 potatoes.
The two machines operate together to get the job done in 1 hour. 
Their combined rate is 300/1 = 300 potatoes per hour.
rate = (amount done)/(time)


The newer machine can do the job alone taking 1.5 hours
rate = (amount done)/(time)
rate = (300 plants)/(1.5 hours)
rate = 200 plants per hour


Since the newer machine can do 200 plants per hour on its own, and the combined rate is 300 plants per hour, then it must mean the older piece of equipment handles 300-200 = 100 plants per hour.
This is under the assumption that the two machines work most efficiently together; meaning that neither machine slows the other down.


Then we have one last set of steps to wrap things up
(rate)*(time) = amount done
time = (amount done)/(rate)
time = (300 plants)/(100 plants per hour)
time = 3 hours
This represents how long it takes the older machine to get the job done if working alone.


Answer: 3 hours
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