a)

$25\%=$ $\frac{25}{100}=\frac{1}{4}$

$50\%=\frac{50}{100}=\frac{1}{2}$

$75\%=\frac{75}{100}=\frac{3}{4}$

$90\%=\frac{90}{100}=\frac{9}{10}$

$b)$

$10\%=\frac{10}{100}=\frac{1}{10}$

$500\%=\frac{500}{100}=5$

$800\%=\frac{800}{100}=8$

$100\%=\frac{100}{100}=1$