Question 12221
Let next test score = x


So average of the 4 tests will be (78+80+90+x)/4 which is (248+x)/4.


This has to be greater than 80 for a B-grade, so 


(248+x)/4 >= 80 is the inequality and is solved as follows...


248+x >= 80*4
248+x >= 320
x >= 320 - 248
x >= 72


jon.