Question 485428: A student is allowed to choose a number at random from counting numbers 60 to 100 inclusive (60, 61, 62, 63,... 97, 98, 99, 100). Using the grading scheme of 60-69=D, 70-79=C, and so on. What is the probability that student gets an A grade?
Answer by Edwin McCravy(20056)   (Show Source):
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60, 61, 62, 63,... 97, 98, 99, 100.
To find the denominator of the probability:
The mistake most people would make here is to assume that there
are 100-60 or 40 possible grades, when there are really 41 possible
grades. The reason it's 41 and not 40 is this:
1. There are 100 numbers from 1 to 100
2. We don't count the numbers from 1 to 59, and there are 59 of them.
3. Therefore the number of numbers from 60 to 100 inclusive is 100-59 or 41.
To find the numerator of the probability
The same kind of mistake most people would make here is to assume that
there are 100-90 or 10 possible A grades, when there are really 11
possible A grades. The reason it's 11 and not 10 is because
1. There are 100 numbers from 1 to 100
2. We don't count the numbers from 1 to 89, and there are 89 of them.
3. Therefore the number of numbers from 90 to 100 inclusive is 100-89 or 11.
So the probability is 11/41.
Edwin
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