A rational number stops and can be turned into a fraction and doesn’t repeat while an irrational number does the opposite
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In order to find an average, you have to add all the numbers and divide by the number of things you added.
120.37 + 108.45 + 114.86 = 343.68
343.68/3 = 114.56
She spent an average of $114.56 each month on groceries.