points x , y , and z lie , in that order , on a straight railroad track . the distance from point x to point y is twice the distance from point y to point z . a train traveled from point x to point z without stopping . the train ' s average speed when traveling from point x to point y was 100 miles per hour and the train ' s average speed when traveling from point y to point z was 75 miles per hour . what was the train ' s average speed , in miles per hour , when traveling from point x to point z ?