Answer
To find the lowest common denominator of two fractions, first find the prime factors of each of the denominators, then take the highest amount of each prime factor and multiply them together.
Work Step by Step
For example, to find the lowest common denominator of $\frac{1}{36}$ and $\frac{1}{40}$ list the prime factorisations of each of the denominators.
36 = 2 x 2 x 3 x 3
40 = 2 x 2 x 2 x 5
Then take the highest amount of each prime factor and multiply them together.
2 x 2 x 2 x 3 x 3 x 5 = 360
The lowest common denominator of $\frac{1}{36}$ and $\frac{1}{40}$ is 360