how to solve a=1/2(b+8)5

I gave xerxes 3 points, but he made a slight mis-type.....we have

a = (1/2) ( b + 8) 5 

a = (5/2) (b + 8) 

a = (5/2)b + (5/2)(8)  

a =(5/2)b + 40/2

a = (5/2)b + 20

$${\mathtt{a}} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}\left({\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}\right){\mathtt{\,\times\,}}{\mathtt{5}}$$                  Rearange (to show what we are doing)

$${\mathtt{a}} = {\frac{{\mathtt{1}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{5}}{\mathtt{\,\times\,}}\left({\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}\right)$$                   Simplify

$${\mathtt{a}} = {\frac{{\mathtt{5}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}\left({\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}}\right)$$                          Expand the brackets

$${\mathtt{a}} = {\frac{{\mathtt{5}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{5}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{5}}$$                            Simplify

$${\mathtt{a}} = {\frac{{\mathtt{5}}}{{\mathtt{2}}}}{\mathtt{\,\times\,}}{\mathtt{b}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{25}}}{{\mathtt{2}}}}$$

The exact value of a depends on b.

Please note: Thanks CPhil for noticing, I have a "typo" in my answer. Please refer to his correct solution.