1 3 5 2
- + - + -- + -
9 7 21 3
The lowest common multiple is 63. The several denominators, when divided
into 63, give 7, 9, 3, 21 respectively, for quotients. Therefore, we
multiply the numerators and denominators of the fractions by 7, 9, 3,
21, and add the numerators to obtain the required sum. The result must
be reduced to a mixed number or to lower terms, if necessary.