Question 816178
Original - modified time = 20 minutes

d=225
speed =x
modified speed = x+20

t=d/r

225/x-225/(x+20) = 1/3 

multiply by x(x+20)

225(x+20)-225x=x(x+20)/3

225x+4500-225x=(x^2+20x)/3
4500*3= x^2+20x

x^2+20x-13500 =0


solve by quadratic formula
a=	1	,	b=	20	,	c=	-13500
							
b^2-4ac=	400	+	54000				
b^2-4ac=	54400						
{{{	sqrt(	54400	)=	233.24	}}}		
{{{x=(-b+-sqrt(b^2-4ac))/(2a)}}}							
{{{x1=(-b+sqrt(b^2-4ac))/(2a)}}}							
x1=(	-20	+	233.24	)/	2		
x1=	106.62						
{{{x2=(-b-sqrt(b^2-4ac))/(2a)}}}							
x2=(	-20	-233.24	) /	2			
x2=	-126.62						
Ignore negative value							
	speed	106.62	mph