0.05(15-x)+0.15(x+0.4)x=4

Solution for 0.05(15-x)+0.15(x+0.4)x=4 equation:

0.05(15-x)+0.15(x+0.4)x=4

We move all terms to the left:

0.05(15-x)+0.15(x+0.4)x-(4)=0

We add all the numbers together, and all the variables

0.05(-1x+15)+0.15(x+0.4)x-4=0

We multiply parentheses

0x^2-0.05x+0x+0.75-4=0

We add all the numbers together, and all the variables

x^2+0.95x-3.25=0

a = 1; b = 0.95; c = -3.25;
Δ = b2-4ac
Δ = 0.952-4·1·(-3.25)
Δ = 13.9025
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0.95)-\sqrt{13.9025}}{2*1}=\frac{-0.95-\sqrt{13.9025}}{2}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0.95)+\sqrt{13.9025}}{2*1}=\frac{-0.95+\sqrt{13.9025}}{2}
$