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what are the common multiples of 2 20 and 16

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we prime factor to know what the primes are, and how many of each.
multiplicand times multiplier equal product. A * B = C.
dividend divided by divisor equal quotient. C / B = A. (when B is not 0).
We decide the number to be prime factored (same as dividend),
we divide by the smallest prime (same as divisor),
and then check the other part (same as quotient) to make sure it is
a whole number (C divided by B equals A with no fraction left over).
 2 = 2 * 1 = 2^1.
20 = 2 * 10 = 2 * 2 * 5 = 2^2 * 5^1.
16 = 2 * 8 = 2 * 2 *4 = 2 * 2 * 2 * 2 = 2^4.
 
we check to see the highest power of each prime that is in any one 
of the least common multiple we are seeking. 
number of 2s multiplied in each ,1 is less than 2 and 4 is highest.
number of 5s multiplied in each,  1 is more than 0. 16 has 5^0 inside it.
5^0 = 1.   i.e. there are no (or zero) 5s multiplied inside 16.
2^4 * 5^1 = 80.
 
80k where k is any whole number would be all the common multiples.
when k = 1, 80. when k =2, 160. et cetera, if you want all multiples.
if you just want the least common multiple (LCM) that is 80.