Gosh I am late to the answer but here we go!
Firstly, lets think about what is given, we have Home, then cyclist at 10 miles apart.
The first twist is 3 minutes/mile, which if solved properly to a speed fraction, would simplify the question.
If it takes 3 minutes to complete 1 mile, then to complete 1 mile, it would take 3 minutes.
Now to find distance covered per minute, simple ratio proportion, and it would be 1/3, which equates to 1/3 miles/minute.
Next up, lets find how much time it would take to complete 10 miles, which is something fixed and the cyclist has to do regardless.
Time = Distance / Speed , = 10/(1/3) = 30 minutes.
Now we only have 18 minutes left, and thinking about how ideally the cyclist is travelling at constant speed, you can half the time, as you need to know how far can the person go one way, as he needs to cover the same distance back.
So in nine minutes, with speed as 1/3, total distance that he can go = 3