DAY 35

Comment

We can replace 8, 10, 21 by 13, 13, 13.

Answer

13.

Readers' methods

Uzoma Isichei, Greenwich SE10:
The average of a number is calculated as
Average = Total / No. of items, and
Total = Average * No. of items.
Therefore the new average is
(13*3 + 13) / 4 = 13.

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