I answered the question before but ill try to explain more. You know after 5 seconds they are going 60km/h, if you want to know how many km/h they are going per second (that would be their acceleration) you would take how fast he is going after those 5 seconds (60km/h) and divide it by how many seconds it takes him to get to that speed (5sec) and your answer would be how many km/h they can get to in one second (their acceleration).