Alex and Brenda both stand at point \(X.\) Alex begins to walk away from Brenda in a straight line at a rate of \(4\)

[Gmat_mission]

Alex and Brenda both stand at point \(X.\) Alex begins to walk away from Brenda in a straight line at a rate of \(4\) miles per hour. One hour later, Brenda begins to ride a bicycle in a straight line in the opposite direction at a rate of \(R\) miles per hour. If \(R > 8,\) which of the following represents the amount of time, in terms of \(R,\) that Alex will have been walking when Brenda has covered twice as much distance as Alex?

A. \(R-4\)

B. \(\dfrac{R}{R+4}\)

C. \(\dfrac{R}{R-8}\)

D. \(\dfrac8{R-8}\)

E. \(2R - 4\)

Answer: C

Source: Manhattan GMAT