In ∆ABC, if a = 18,

Given,∠C = 90°, a = 18, b = 24 and c = 30

According to sine rule, $\frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C}$.

$\Rightarrow \frac{c}{\sin C}=\frac{a}{\sin A}$

$\Rightarrow \sin A=\frac{a \sin C}{c}$

$=\frac{18 \times \sin \left(90^{\circ}\right)}{30}$

$=\frac{18}{30}$

$=\frac{3}{5}$

Also, $\frac{b}{\sin B}=\frac{c}{\sin C}$

$\Rightarrow \sin B=\frac{b \sin C}{c}$

$=\frac{24 \sin 90^{\circ}}{30}$

$=\frac{24}{30}$

$=\frac{4}{5}$

and,

$\sin C=\sin 90^{\circ}=1$