The denominators are $5$ and $10.$ Since $10$ is a multiple of $5,$ we can make a common denominator of $\color{blue}10.$

We split the shape on the left of the minus sign into $2$ parts vertically:

Now, the shape to the left shows $\dfrac{8}{\color{blue}10}$ and the shape to the right shows $\dfrac{3}{\color{blue}10}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 8 -3 = {\color{red}{5}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{4}{5} - \dfrac{3}{10} = \dfrac{8}{\color{blue}10} - \dfrac{3}{\color{blue}10} = \dfrac{\color{red}5}{\color{blue}10}. \]

Finally, we can simplify this fraction by dividing the numerator and denominator by $5$: \[ \dfrac{5}{10}=\dfrac{5\div 5}{10 \div 5}=\dfrac{1}{2}. \]

The denominators are $4$ and $8.$ Since $8$ is a multiple of $4$, we can make a common denominator of $\color{blue}8.$

We split the shape on the right of the minus sign into $2$ parts vertically:

Now, the shape to the left shows $\dfrac{7}{\color{blue}8}$ and the shape to the right shows $\dfrac{6}{\color{blue}8}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 7-6 = {\color{red}{1}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{7}{8} - \dfrac{3}{4} = \dfrac{7}{\color{blue}8} - \dfrac{6}{\color{blue}8} = \dfrac{\color{red}1}{\color{blue}8}. \]

The denominators are $3$ and $9.$ Since $9$ is a multiple of $3$, we can make a common denominator of $\color{blue}9.$

We split the shape on the left into $3$ parts horizontally:

Now, the shape to the left shows $\dfrac{6}{\color{blue}9}$ and the shape to the right shows $\dfrac{4}{\color{blue}9}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 6-4 = {\color{red}{2}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{2}{3} - \dfrac{4}{9} = \dfrac{6}{\color{blue}9} - \dfrac{4}{\color{blue}9} = \dfrac{\color{red}2}{\color{blue}9}. \]

The denominators are $8$ and $2.$ Since $8$ is a multiple of $2$, we can make a common denominator of $\color{blue}8.$

We split the shape on the right into $4$ parts horizontally:

Now, the shape to the left shows $\dfrac{5}{\color{blue}8}$ and the shape to the right shows $\dfrac{4}{\color{blue}8}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 5-4 = {\color{red}{1}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{5}{8} - \dfrac{1}{2} = \dfrac{5}{\color{blue}8} - \dfrac{4}{\color{blue}8} = \dfrac{\color{red}1}{\color{blue}8}. \]

The denominators are $3$ and $4.$ So, we can make a common denominator of $3 \times 4 = \color{blue}12.$

We split the shape on the left of the minus sign into $4$ parts vertically, and we split the shape on the right into $3$ parts horizontally:

Now, the shape to the left shows $\dfrac{8}{\color{blue}12}$ and the shape to the right shows $\dfrac{3}{\color{blue}12}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 8 -3 = {\color{red}{5}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{2}{3} - \dfrac{1}{4} = \dfrac{8}{\color{blue}12} - \dfrac{3}{\color{blue}12} = \dfrac{\color{red}\fbox{$\,5\,$}}{\color{blue}12}. \]

So, the missing number is $\color{red}5.$

The denominators are $5$ and $2.$ So we can make a common denominator of $5\times 2 = \color{blue}10.$

We split the shape on the left of the minus sign into $5$ parts horizontally, and we split the shape on the right into $2$ parts vertically:

Now, the shape to the left shows $\dfrac{5}{\color{blue}10}$ and the shape to the right shows $\dfrac{2}{\color{blue}10}.$

Next, we subtract the number of shaded parts on the right from the number on the left. \[ 5-2 = {\color{red}{3}} \] This gives the picture shown below.

Therefore, we have \[ \dfrac{1}{2} - \dfrac{1}{5} = \dfrac{5}{\color{blue}10} - \dfrac{2}{\color{blue}10} = \dfrac{\color{red}\fbox{$\,3\,$}}{\color{blue}10}. \]

So, the missing number is $\color{red}3.$