Javier walks from his house to the zoo at a constant rate. After walking 0.75 mile, he meets his brother, Raul, and they continue walking at the same constant rate. When they arrive at the zoo, Javier has walked for 0.5 hour and Raul has walked for 0.2 hour. Which equation can be used to find r, the rate in miles per hour at which the brothers walked to the zoo?
A 0.2r = 0.5r
B 0.2r + 0.75 = 0.5r
C 0.5r + 0.75 = 0.2r
D 0.75r = 0.5r + 0.2r