Question 305798
Given: {{{200/(R+10)= (200/R) -1}}}
Multiply everything by (R+10)
{{{200 = 200 + 2000/R - R - 10}}} Combine like terms
{{{200 = 190 + 2000/R - R}}}
Multiply Everything by R
{{{200R = 190R + 2000 - R^2}}}
Subtract 200R from both sides
{{{0 = -10R + 2000 - R^2}}}
Use the quadratic equation
*[invoke quadratic "x", -1, -10, 2000]

The quadratic equation gives you two answers: -50, & 40.  Since Amy could not have been driving at a negative rate, 40 is the only answer.