Question 141660
The average of your first 8 scores, A, is the sum of the 8 scores divided by 8. 
The average of your 12 scores,B, is the sum of the 12 scores divided by 12.
A=SUM8/8
SUM8=8A
SUM8=8(0.865)
SUM8=6.92
B=SUM12/12
SUM12=12B
SUM12=12(.90)
SUM12=10.8
SUM12=SUM8+NINE+TEN+ELEVEN+TWELVE
where NINE, TEN, ELEVEN, and TWELVE are test scores 9,10,11, and 12. 
SUM12=SUM8+NINE+TEN+ELEVEN+TWELVE
10.8=6.92+NINE+TEN+ELEVEN+TWELVE
NINE+TEN+ELEVEN+TWELVE=3.88
(NINE+TEN+ELEVEN+TWELVE)/4=3.88/4
(NINE+TEN+ELEVEN+TWELVE)/4=0.97

You need to average a 97% on the next four tests to get 90% average.