The area of what is not shaded should be 20 Square Units. If you want to know how to do that you just multiply the base of the triangle by the height of the triangle then multiply that number by 0.5 and you should get your answer.
By using LCF (Least common factor), the answer would be 24. If you count by sixes and eighteen, the first number they have the same is 24. 6,12,18,24 8,16,24