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Problem Twenty-Five Hint
The overarching idea is still average speed: total distance over total time.
You have to represent distance with fractions. For example, if it said he traveled one fourth of the distance you would write that as: 1/4x.
You will need to use two times here: t1 and t2, which add up to the total time of the trip.
You will have three initial equations from the given equation that have the following variables: x, v, t1, and t2. You'll have to figure out a way to solve two of them for a certain variable (I'm not going to tell you which), and then substitute into the third equation. When you do that, you will be able to cancle out the x's, which means you will only be left with one variable: v, which is the speed over the remaining part of the distance.
Enjoy!