(This is from CTK exchange)
A column of soldiers is 25 miles long and they march 25 miles a day. One morning a messenger started at the rear of the column with a message for the guy up front. The messenger began to march and gave the message to the guy up front and then returned to his position by the end of the day. Assume that the messenger marched at the same rate of speed the whole time. How many miles did the messenger march?
Ok, As I said earlier the distance covered by messenger has to be (50 + 2d) and solving it using algebra the way similar to what TomM did, the answer comes out to be the one submitted by fussano. 75 miles. Good logic fussano. But your explanation was not proper. What I feel is. The messenger must march a (minimum) distance equal to twice the length of the column of soldier, i.e ofcourse considering that the troops are stationary and not moving. But since the troop is moving he has to cover that extra distance moved by the troop in that day. So for example if the column of the troop is 50 miles and they march 45 miles a day, then I believe the messenger must have travelled a distance of 100 + 45 in that day.
I have not verified the above logic using algebra. So I am not sure. But TomMs solution is full proof.