On Monday, $10 \%$ of the students at Dunkley S.S. were absent and $90 \%$ were present. On Tuesday, $10 \%$ of those who were absent on Monday were present and the rest of those absent on Monday were still absent. Also, $10 \%$ of those who were present on Monday were absent and the rest of those present on Monday were still present. What percentage of the students at Dunkley S.S. were present on Tuesday?
(A) $81 \%$
(B) $82 \%$
(C) $90 \%$
(D) $91 \%$
(E) $99 \%$
Suppose that there are 1000 students at Dunkley S.S.

On Monday, there were thus 100 students absent and 900 students present.

On Tuesday, $10 \%$ of the 900 students who were present on Monday, or $0.1(900)=90$ students, were absent. The remaining $900-90=810$ students who were present on Monday were still present on Tuesday.

Similarly, $10 \%$ of the 100 students who were absent on Monday, or $0.1(100)=10$ students, were present on Tuesday. The remaining $100-10=90$ students who were absent on Monday were still absent on Tuesday.

Thus, there were $810+10=820$ students present on Tuesday, or $\frac{820}{1000} \times 100 \%=82 \%$ of the whole student population.