Convert the given common fractions to decimal fractions and leave your answer in 1dp.

$$\frac{12 \div 4}{100 \div 4} = \frac{3}{25}$$

The common fraction is $\boxed{\frac{3}{25}}$.

b) 0.05

$$\frac{5 \div 5}{100 \div 5} = \frac{1}{20}$$

The common fraction is $\boxed{\frac{1}{20}}$.

c) 0.9

The common fraction is $\boxed{\frac{9}{10}}$.

Part 2: Convert to decimal fraction and leave your answer in 1 dp.

a) $\frac{2}{3}$

The second decimal place is 6, so we round up the first decimal place.

$$0.666... \approx 0.7$$

The decimal fraction to 1 dp is $\boxed{0.7}$.

b) $\frac{2}{9}$

The second decimal place is 2, so we keep the first decimal place as it is.

$$0.222... \approx 0.2$$

The decimal fraction to 1 dp is $\boxed{0.2}$.

c) $2 \frac{1}{4}$

The second decimal place is 5, so we round up the first decimal place.

$$2.25 \approx 2.3$$

The decimal fraction to 1 dp is $\boxed{2.3}$.