Multiply both sides of the equation by 20+a. On the right hand side, you will be both multiplying 3000 by 20+a and dividing by 20+a, so essentially, you are multiplying 3000 by 1.
The next step, they divide both sides by 100. 100*(20+a) divided by 100 is 20+a. 3000 divided by 100 is 30.