Question 169315
factor each number into primes

20=2*2*5
36=2*2*3*3
45=3*3*5

now take the largest amount of each prime that occurs in this case 2*2 is the largest occurence of 2 , 3*3 is the largest occurence of 3 and a single 5 is the largest occurence of 5

now multiply all those together to get lcm {{{2*2*3*3*5=180}}}