If $a=2$ and $b=5$, which of the following expressions has the greatest value?
(A) $\frac{a}{b}$
(B) $\frac{b}{a}$
(C) $a-b$
(D) $b-a$
(E) $\frac{1}{2} a$
Evaluating each of the five given possibilities with $a=2$ and $b=5$,
(A) $\frac{a}{b}=\frac{2}{5}$
(B) $\frac{b}{a}=\frac{5}{2}$
(C) $a-b=2-5=-3$
(D) $b-a=5-2=3$
(E) $\frac{1}{2} a=\frac{1}{2}(2)=1$

The largest of these five is 3 , or $(\mathrm{D})$.

ANswer: (D)