Solve (82+71+83+91+78+x)/6 = 79.5 for x :
=> (82+71+83+91+78+x) = 6*79.5 = 477
=> (82+71+83+91+78+x) = 477 = 405 + x
=> x = 477 - 405 = 72
as above
19.February.2015

If the sixth number is 72 then the mean of the six numbers will be 79.5