How do you do least common multiple fractions?

When it's fractions, we call it the lowest common denominator, or LCD.
Example: 1/30 and 1/42
Factor the denominators.
2 x 3 x 5 = 30
2 x 3 x 7 = 42
Combine the factors, eliminating duplicates.
2 x 3 x 5 x 7 = 210, the LCD