Question 468315
To vary directly means there is a direct relationship between 2 variables.
As speed increases so does stopping distance, etc.
However, how much of an effect speed has on stopping distance is unknown, it is represented as a constant scale factor k.
{{{d = k*r^2}}}
Given a speed and corresponding stopping distance, you can solve for k.
{{{140 = k*50^2}}}
{{{140 = k*2500}}}
{{{k = 140/2500 = 0.056}}}
Now substitute this value into the equation
{{{d = .056r^2}}}
Now given any speed, you can solve for stopping distance
For r = 80
{{{d = .056*80^2}}}
{{{d = .056*6400}}}
{{{d = 358.4}}}