Final answer:
Mateo decreased his time for the mile by 20%.
Step-by-step explanation:
To calculate the percentage decrease in Mateo's running time, we need to find the difference between his old time and new time, and then divide it by his old time.
Step 1: Calculate the difference: 10 minutes - 8 minutes = 2 minutes.
Step 2: Divide the difference by the old time: 2 minutes / 10 minutes = 0.2.
Step 3: Convert the decimal to a percentage by multiplying by 100: 0.2 * 100 = 20%.
Therefore, Mateo decreased his time for the mile by 20%.