$5÷3=\frac{5}{3}(元),7÷4=\frac{7}{4}(元),8÷5=\frac{8}{5}(元)$
$\frac{5}{3}=\frac{100}{60},\frac{7}{4}=\frac{105}{60},\frac{8}{5}=\frac{96}{60}$
$因为\frac{96}{60}\lt \frac{100}{60}\lt \frac{105}{60},所以\frac{8}{5}<\frac{5}{3}<\frac{7}{4}$
$所以在丙商店最便宜,在乙商店最贵。$