The decimal 0.875 is the equivalent of 7/8 as a fraction. To determine this, take 875/1000, and reduce the fraction as much as possible. Divide the numerator and denominator by 25 to get 35/40. Reduce the fraction by 5 again to get 7/8.
The greatest common divisor, or greatest common factor, between 875 and 1000 is 125, or the same amount the fraction was reduced by the numbers 25 times 5. The greatest common divisor is the largest number that both portions of the fraction can be divided by evenly. If the greatest common factor is unknown, start with the easiest factor, and divide both numbers by that term to lower the numerator and denominator.