Running Dog

Two people - Mick and Goof, 100 meters apart, start walking towards each other with constant speeds of 2m/s. At the same time Mick's dog starts running back and fourth between them with constant speed of 6m/s until Mick and Goof meet. How much distance does the dog cover in total?

Mick and Goof will meet after 100/(2+2)=25 seconds. Therefore the dog will run for 25 seconds and will cover 6x25=150 meters in total.