The value of 0.444… + 0.222… − 0.111… (each of the digits repeating for ever) is:

A single repeating digit d gives the fraction d/9, so the three numbers are 4/9, 2/9 and 1/9. Then 4/9 + 2/9 − 1/9 = 5/9, and 5/9 = 0.555… . Forgetting to subtract the last term would give 7/9 = 0.777… .