Help with some algebra 2 word problems.

1. A commercial jet can fly from San Francisco to Dallas in 3 hours. A private jet can make the same trip in 3.5 hours. If the two planes leave San Francisco at noon, after how many hours is the private jet twice as far from Dallas as the commercial jet? The answer is 21/8 hours or 2 and 5/8 hours, just need to know how to get there. >.<

2.A car radiator is filled with 5 liters of a 25% antifreeze solution. How many liters must be drawn off and replaced by a 75% antifreeze solution to leave the car radiator fillers with a 55%antifreeze solution? Don't have the answer to this one.

3.The rail line between two cities consist of two segments, one 96 km longer than the other. A passenger train averages 60 km/h over the shorter segment, 120 km/h over the longer, and 100 km/h for the entire trip. How far apart are the cities? The answer is 160 km, just need to see how to get there.

thanks in advance : D