So you have unknown value (n)
n × 30 = 20
You can divide both sides by 30, to get n to be isolated;
n ×30 / 30 = 20/30
n times 30 divides 30 is same as n, because these 30s cancel each other.
So you have n = 20/30, which can be reduced to n = 2/3
So 2/3 times 30 equals 20. Hope this helps.