6 test ---- 82
1 test ---- x
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x = 82/6 = 13,66 so from this result that the average of one test from your first 6 tests is equal 13,66 bc. this 13,66 multiplied by 6 will result this 82
82-80,5 = 1,5 so from this that the average of your seventh test was 1,5