A puzzle from R.M. Abraham’s Diversions & Pastimes, 1933:

A prisoner escapes from Dartmoor Prison and has half-an-hour’s start of two warders and a bloodhound who race after him. The warders’ speed is 4 miles per hour; the dog’s 12 miles per hour, but the prisoner can only do 3 miles per hour. The dog runs up to the prisoner and then back to the warders, and so on back and forth until the warders catch the prisoner. How far does the dog travel altogether?