of 20 and 36
Hi ckica,
I guess you're looking for the least common multiple, right?
Factor each value using prime factors:
$\displaystyle 20=2^2 \cdot 5$
$\displaystyle 36=2^2 \cdot 3^2$
Now multiply all the different factors using their greatest exponent:
$\displaystyle LCM=2^2 \cdot 3^2 \cdot 5=180$
It is if you're looking for the least common multiple, which I assumed.
List the multiples of 20 and 80. You see that 180 is the first multiple common to both. There is an infinite numbers of common multiples, but 180 is the least common multiple (LCM).
20, 40, 60, 80, 100, 120, 140, 160, 180
36, 72, 108, 144, 180