(25-y)y=100

Solution for (25-y)y=100 equation:

(25-y)y=100

We move all terms to the left:

(25-y)y-(100)=0

We add all the numbers together, and all the variables

(-1y+25)y-100=0

We multiply parentheses

-1y^2+25y-100=0

a = -1; b = 25; c = -100;
Δ = b2-4ac
Δ = 252-4·(-1)·(-100)
Δ = 225
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{225}=15$
$y_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(25)-15}{2*-1}=\frac{-40}{-2}
=+20
$
$y_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(25)+15}{2*-1}=\frac{-10}{-2}
=+5
$