To add or subtract fractions with different denominators, the denominators must be made common. For 3/4 + 1/2, the denominator of 1/2 is multiplied by 2 to make the common denominator 4. The sum is 5/4 or 1 1/4. For 7/10 - 3/5, the denominator of 3/5 is multiplied by 2 to make the common denominator 10. The difference is 1/10.