Final answer:
The athlete runs 12 miles on the day they cover 40% of their weekly mileage of 30 miles.
Step-by-step explanation:
The athlete runs 30 miles a week in three runs. To find out how long the athlete runs on the day they cover 40% of the weekly mileage, we will calculate 40% of 30 miles. This is done by multiplying the total mileage (30 miles) by the percentage (0.40) as follows:
30 miles × 0.40 = 12 miles
Therefore, on the day the athlete runs 40% of their weekly mileage, they run 12 miles.