答案：2. $0.75×0.6$
$=(0.01×75)×(0.1×6)$
$=(0.01×0.1)×(75×6)$
$=0.001×450$
$=0.45$
$\frac{11}{2}×\frac{5}{4}$
$=(\frac{1}{2}×11)×(\frac{1}{4}×5)$
$=(\frac{1}{2}×\frac{1}{4})×(11×5)$
$=\frac{1}{8}×55$
$=\frac{55}{8}$
$\frac{7}{5}÷\frac{21}{8}$
$=(\frac{1}{5}×7)÷(\frac{1}{8}×21)$
$=(\frac{1}{5}÷\frac{1}{8})×(7÷21)$
$=\frac{8}{5}×\frac{1}{3}$
$=\frac{8}{15}$