What is the number?

If 20 is added to one-third of a number, the result is the double of the number. What is the number?

Let N be the number. 

So 20+(1/3)N=2N

Rearranging like terms;

2N-(1/3)N = 20
ie (5/3)N=20
=>N= 20*3/5 = 12

The number is 12.