I know how to find the exact value of $\sin \frac{\pi}{10}$ using double and triple angle formulas and the fact that $\frac{5\pi}{10}=\frac{\pi}{2}$ but it maybe too complicated for high school students. Is there an easier way that I do not see? The answer is $0.5$.
Unfortunately I did not see a clear answer among posted answers to my question but thanks to @labbhattacharjee, the main idea is to multiply and divide by $2 \cos 18°$.
Thus $\large{\sin 54°-\sin 18°=\frac{2 \sin 54°\cos 18°-2 \sin 18°\cos 18°}{2 \cos 18°}=\frac{\sin 72° + \sin 36°-\sin 36°}{2 \cos 18°}=\frac{\sin 72°}{2\cos 18°}=0.5}$