x*0.8x=1280

Solution for x*0.8x=1280 equation:

x*0.8x=1280

We move all terms to the left:

x*0.8x-(1280)=0

Wy multiply elements

0x^2-1280=0

We add all the numbers together, and all the variables

x^2-1280=0

a = 1; b = 0; c = -1280;
Δ = b2-4ac
Δ = 02-4·1·(-1280)
Δ = 5120
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{5120}=\sqrt{1024*5}=\sqrt{1024}*\sqrt{5}=32\sqrt{5}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-32\sqrt{5}}{2*1}=\frac{0-32\sqrt{5}}{2}
=-\frac{32\sqrt{5}}{2}
=-16\sqrt{5}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+32\sqrt{5}}{2*1}=\frac{0+32\sqrt{5}}{2}
=\frac{32\sqrt{5}}{2}
=16\sqrt{5}
$