I think you mixed least common multiples and greatest common divisors. The multiples are always number greater than the two given.

There is a neat way for working out least common multiples and greatest common divisors: Prime factorization. Take 16 and 40, and express them as products of numbers, non divisible by others: 16=2*8=2^4 (power) , and 40=5*8=5*2^3. For the least common multiple [16,40], we have that products common in both places get out of the brackets:

[16,40]=[2^4,5*2^3]=2^3*[2,5] (2^3 is common in both places)

=8*[2,5]=8*2*5=80

( 'cause 2 and 5 haveno common divisors, so bracket becomes product)

For the greatest common divisor, (16,40) the same considerations hold, accept thatwhen two numbers in the parentheses have no common divisors, the result is 1.We have then

(16,40)=(2^4,5*2^3)=2^3*(2,5)=8*1=8.