After five tests allison's average score was 88. after six tests her average score had increased to 90. what was her score on the sixth test? show your work.

My favorite thing to do in my pastimes in calculate what I can get on a final test to keep an average. 

First, we have to find out what the total of the 5 tests was. This is done by multiplying the average score by 5 (because in calculating the mean/average, we would originally divided all the scores added up by the number of tests):
88*5 = 440

Then we have to do the following to find the average score (which we know already:
90= (440+x)/6
540=440+x
x=540-440
  =100

Her score on the sixth test was 100