n cars is to t hours as 1 car is to ??? hours.
Let X be the answer.
n cars is to t hours as 1 car is to ??? hours.
n is to t as 1 is to X
n:t = 1:X
n over t equals one over X
Cross-multiply:
Joseph can paint 1 car in hours.
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Here's an easy way to think it through to get the hang of it:
If he can paint 2 cars in t hours, it will only take him 1/2 as long to paint just 1 car, or t/2 hours.
If he can paint 3 cars in t hours, it will only take him 1/3 as long to paint just 1 car, or t/3 hours.
If he can paint 4 cars in t hours, it will only take him 1/4 as long to paint just 1 car, or t/4 hours.
If he can paint 5 cars in t hours, it will only take him 1/5 as long to paint just 1 car, or t/5 hours.
etc.
So it's easy to go from there to:
If he can paint n cars in t hours, it will only take him 1/n as long to paint just 1 car, or t/n hours.
Edwin
