To find a common denominator for the fractions \( \frac{1}{4} \) and \( \frac{3}{5} \), we need to calculate the least common multiple (LCM) of the denominators, which are 4 and 5.
The multiples of 4 are:
- 4, 8, 12, 16, 20, 24, ...
The multiples of 5 are:
- 5, 10, 15, 20, 25, ...
The smallest multiple that appears in both lists is 20.
So, the least common denominator (LCD) of \( \frac{1}{4} \) and \( \frac{3}{5} \) is 20.