If the arc on a particular circle has a length of 12 units, and the circumference of the circle is 36, which is the degree measure of the arc?

So the answer will be 120° and here is why: 

We can say that x∘=x360

And then 

36(x360)=12

36(x360)=12

x10=12

x=10⋅12=120∘

what is the original number? you forgot to add it.