Prime factors of $15 = 3 × 5 $

Prime factors of $20 = 2^2 × 5$

Prime factors of $8 = 2^3$

Using exponents instead, multiply together each of the prime numbers with the highest power to find the LCM

$$2^3 × 3^1 × 5^1 = 120$$

So LCM(15,20,8) = 120