the average of eighty-five and a number m is ninety

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the average of eighty-five and a number m is ninety

Average of 2 numbers means we add both numbers and divide by 2:
(85 + m)/2 = 90

Cross multiply:
m + 85 = 90 * 2
m + 85 = 180

To solve this equation for m, we type it in our math engine and we get:
m = 95