Okay I am having trouble setting up this math word problem.

In Mr. Black's class 20 people took a biology test and had an average of 88%. Two more took the test later and made an average of 55% What is the average of the entire class?

Also, I have gotten an answer for this one but not sure if it is right so I guess how do you set this one up:

When 100 students took a test the average score was 77.1 Two more students took the test. The sume of their scores was 125. What is the new average?