Jacob decided to ride his bicycle across the country during his 2-month summer vacation. the route he took from washington, dc to portland, oregon, covered a total of 2800 miles. to keep his mind occupied during some of the long flat stretches of countryside, he often did algebra problems in his head. one day, for example, he determined that his distance from his starting point was exactly 100 miles more than three times the distance remaining until the finishing point. how far was jacob from the finishing point of his journey when he made that calculation? start your work by defining the unknown quantities in terms of one variable. show all work and write a complete answer, including units