I have the numbers $a = 120, b = 144$. So if I prime them I get $120 = 5\times3\times2\times2\times2$ and $b = 144 = 2\times3\times3\times2\times2\times2$.
I am looking for the lowest number that is divisible by both $120$ and $144$ . Do you know how to find that ?
lcm(120, 144) = 720. You can check this on Wolfram-Alpha for instance.
To find them: Write out the minimal number of primes in the decomposition for each of the numbers, that is here $2^4 \cdot 3^2 \cdot 5 = 720$.