for finding difference or sum of unlike fractions ( fractions with different denominators) then we need to make their denominators same ...so for that we take their denominators LCM ... 7/4 - 1/8 (LCM of 4 and 8 is 8) => 7/4 X 2 - 1/8 => 14/8 - 1/8 = 13/8 ..
