Mean of five numbers is 20, and mean of the remaining four numbers after excluding one is 18.

Calculate the change in mean ($20 - 18 = 2$) and multiply by the remaining count ($4$), then add the new mean ($18 + (2 \times 4) = 26$ is incorrect; rather: original mean + change $\times$ remaining = $20 + (2 \times 4) = 28$).

Students often subtract the new sum from the original sum but miscount the number of items remaining or forget that the excluded number contributes to the original mean.

This concept of 'sum adjustment' is fundamental for questions involving 'replacement of members' in groups, a common topic in Data Interpretation and Average-based aptitude tests.