What is the sum of the infinite geometric series?

This website explains a part of this --> https://www.quora.com/What-is-1-3-1-9-1-27-1-81-1-243

This means the value of 40/3 + 40/9 +... = 40/2, which equals 20. 

This means your answer is 120 + 40 + 20, which gives you the answer of 180 :)