Average temperature of a week is 30°C. If the average of first 4 days was 31°C, then average temperature of the remaining days will be:

Total temperature of the week = 30°C × 7 = 210°C
Total temperature first four day = 31°C × 4 = 124°C
Average temperature of remaining day
$$\eqalign{ & = \frac{{{{210}^ \circ }{\text{C}} - {{124}^ \circ }{\text{C}}}}{3} \cr & = \frac{{{{86}^ \circ }{\text{C}}}}{3} \cr & = {28.67^ \circ }{\text{C}} \cr} $$