So average is of the first 6 tests is 82 well that means
(test1 + test2+ test3+ test4 + test5+test6)/6 = 82
so now let do some cross multiplliying
test1 + test2 + test3 + test 4 + test5 +test6 = 82*6
test1 + test2 + test3 + test 4 + test5 +test6 = 492
now lets see if we can find that 7th test score
(test1 + test2 + test3 + test4 + test5 +test6 + test7)/7 = 80.5
So look we found test1 + test2 + test3 + test4 + test5 +test6 to be 492 so lets substitute.
(492 + test7 )/7 = 80.5
test7 = (80.5*7)-492 = 71.5
