Ten times the sum of half a number and 6 is 8

Even though we don't know the number yet, we need to give it some kind of label
so that we can work with it.  We can call it anything.  I'd like to call it ' G '.

The number we're looking for is ' G '.

Half of it is  1/2 G

The sum of that and 6 is          1/2 G + 6

Ten times that is              10 (1/2 G + 6)

The question says that's 8.
                                        So       10 (1/2 G + 6) = 8

Divide each side by  10 :                1/2 G + 6  =  0.8

Subtract  6  from each side:            1/2 G  =  - 5.2

Multiply each side by  2 :                        G  =  - 10.4

That seems like a weird answer.  We should check it.

The number:                                         -10.4
Half the number:                                   - 5.2
The sum of half the number and 6:      -5.2 + 6 = +0.8
Ten times that sum:                                                  8

Yep !  By golly, sho nuff, dV-dT and E to the X !  
Everything checks out, and the mystery number is      - 10.4