It depends on what the other unit you are rating is. For example, if it takes 3 hours to make 315 dollars, or if it takes 315 minutes to complete one course.
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a unit rate
A unit rate means a rate for one of something. We write this as a ratio with a denominator of one.
450miles in 3hours and 45min
Write as a unit rate . $600 in 5 weeks
It is: 315 = CCCXV
You rename the rate such that either the numerator, or the denominator is 1. It is usually the denominator, but can be the numerator.
A rate of measurement is the number of measurements made per unit of time.
Write each rate as a unit rate 70 miles in 2 hours
315 = three hundred fifteen.
57 mph
Divide 54 words by 3 minutes to obtain this unit rate, 18 words per minute.