arithmetic - Geometric series word problem help?

Philip received 75 points on a project for school. He can make changes and receive two-tenths of the missing points back. He can make corrections as many times as he wants. Create the formula for the sum of this geometric series and explain your steps in solving for the maximum grade Philip can receive. Identify this as converging or diverging.

• The formula would be $$S_n= \frac{a_1-a_1r^n}{ 1-r}$$

  
  
  

  but am I missing something and how do I find $n$? Is it diverging?