36 = 2*2*3*3
45 = 5*7
63 = 3*3*7
80 = 2*2*2*2*5
Then take each unique factor (2, 3, 5, and 7) and raise each the greatest power it has in any of the input numbers (2^4, 3^2, 5^1, 7^1) and multiply them, and that is your answer.
2*2*2*2*3*3*5*7 = 5040