AHSME 1951 Problem 50

Tom, Dick and Harry started out on a $100$-mile journey. Tom and Harry went by automobile at the rate of $25$ mph, while Dick walked at the rate of $5$ mph. After a certain distance, Harry got off and walked on at $5$ mph, while Tom went back for Dick and got him to the destination at the same time that Harry arrived. The number of hours required for the trip was: $\textbf{(A)}\ 5 \textbf{(B)}\ 6 \textbf{(C)}\ 7 \textbf{(D)}\ 8 \textbf{(E)}\ \text{none of these answers}$