The average of a number and double the number is 25.5

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Let x equal "a number".

Double the number is 2x.

The average is (x + 2x)/2

Combine the terms in the numerator:
3x/2

The word is means equal to, so we set 3x/2 equal to 25.5
3x/2 = 25.5

Cross multiply the 2:
3x = 51

Divide each side by 3
x = 17