This is yet another simple problem from Henry Dudeney.
“57. TIMING THE CAR
“I was walking along the road at three and a half miles an hour,” said Mr. Pipkins, “when the car dashed past me and only missed me by a few inches.”
“Do you know at what speed it was going?” asked his friend.
“Well, from the moment it passed me to its disappearance round a corner I took twenty-seven steps and walking on reached that corner with one hundred and thirty-five steps more.”
“Then, assuming that you walked, and the car ran, each at a uniform rate, we can easily work out the speed.” ”
See Timing the Car for a solution.