SOLUTION:
If we know the LCM of 4 and 5, which is 20, then to find the LCM of 40 and 50 that will be the LCM of 4 and 5 multiplied by 10 (i.e. 20 x 10 = 200) because both 4 and 5 are multiplied by 10 to get 40 and 50, so the LCM of of 4 and 5 must be multiplied by 10 for us to get the LCM of 40 and 50.
In conclusion;
LCM of 4 and 5 = 20
LCM of 40 (i.e. 4 x 10) and 50 (i.e. 5 x 10) = 200 (i.e. 20 x 10).