(a+b)(5) = 20

Multiply both sides by (2) to clear of fractions:

(2)(a+b)(5) = (2)20

The (2) and the  multiply to give 1 and (2) times the 20 
multiply to give 40.  So we now have

(1)(a+b)(5) = 40

We can remove the (1)

   (a+b)(5) = 40

Next we divide both sides by 5:

   = 

Now we can cancel the (5) and the 5, and 40 divided by 5 gives 8.
So we have:

   = 8

  (a+b) = 8

Edwin