Kristen completed 42 km. We will imagine that she ran all 6 hours. => 9 * 6 =54 km which is more than she actually completed. So we will find a 9 Multiple less than 42 km and that will be 36 km. (9 * 4 = 36 km completed by running) and the rest until 42 were completed by walking (3 * 2 = 6; 6 + 36 =42).
