3/20, or 0.15 of the shirts each evening
1 - 1/4 = 3/4
(3/4) / 5 = (3/4) / (5/1) = (3/4) x (1/5) = 3/20 = 0.15
If I were solving this equation, I would put x^2-20x+(100)=(10). Since this is in standard form, you could change this into vertex form by making the equation y=(x-10)^2 -10.