Jacob decided to ride his bicycle across the country during his 3-month summer vacation. The route he took from Washington, DC to Portland, Oregon,...

Jacob decided to ride his bicycle across the country during his 3-month summer vacation. The route he took from Washington, DC to Portland, Oregon, covered a total of 3420 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 the distance remaining until the finishing point was exactly 36 miles more than half of the distance from his starting point. How far was Jacob from the finishing point of his journey when he made that calculation? 

I need to define the unknown quantities in terms of variable.