Known and understood law: .
Use this to fill in a data table if that helps, as I did.
Notice from the basic rule, .
(meters per sec) ( sec) (meters)
SPEED TIME DISTANCE
walking with r+1.5 200/(r+1.5) 200
not walking r 200/r 200
DIFFERENCE 30
Solve for r.
The equation then comes from the information in the table. Matching the description.