10% of the race is 41.2 miles. Why does this make sense? How could you use that to calculate 20% of the race?
How could you find the length of 1% of the race? Can you combine that with your answer from part (a) to find 21% of the race?
They have ridden 20% + 21% = 41% of the race. What percentage is left?
You can do this by calculating another percentage of the race.
Or you can do it by subtracting the miles they have already ridden from the total length of the race.
Which method do you prefer?
You can try doing both to check that they get the same answer!