The obvious answer to this would be 10.
The most simple way to do it would be taking out 3 x from each side of the equation and do 20 = 10 + _
Another way would be to actually solve it.
Let us put into the situation that the unknown variable would be y. The equation then would look like this.
3 x 20 = 3 x (10 + y)
Then solve
3 x 20 = 3 x (10 + y)
-------- ---------------
3 3
20 = 10 + y
20 - 10 = y
10 = y
10 would be the answer