Which of the following is not equal to $\frac{15}{4}$ ?
(A) 3.75
(B) $\frac{14+1}{3+1}$
(C) $\frac{3}{4}+3$
(D) $\frac{5}{4} \times \frac{3}{4}$
(E) $\frac{21}{4}-\frac{5}{4}-\frac{1}{4}$
Since $\frac{14+1}{3+1}=\frac{15}{4}$ and $\frac{21}{4}-\frac{5}{4}-\frac{1}{4}=\frac{16}{4}-\frac{1}{4}=\frac{15}{4}$, then answers (B) and (E) both simplify to $\frac{15}{4}$. Written as a mixed fraction, $\frac{15}{4}$ is equal to $3 \frac{3}{4}$.

Since $3.75=3 \frac{3}{4}=3+\frac{3}{4}$, then answers (A) and (C) both simplify to $3 \frac{3}{4}$ and thus are equivalent to $\frac{15}{4}$.

Simplifying answer (D), $\frac{5}{4} \times \frac{3}{4}=\frac{5 \times 3}{4 \times 4}=\frac{15}{16}$.

Thus, $\frac{5}{4} \times \frac{3}{4}$ is not equal to $\frac{15}{4}$.

ANSWER: (D)