Final answer:
Monica averaged 10 miles per week last month.
Step-by-step explanation:
To find out how much Monica averaged last month, we need to calculate 80% of her average mileage this month and subtract it from her average mileage this month.
Let's represent Monica's average mileage last month as x. We have the equation x + 0.8x = 18.
Simplifying the equation, we get 1.8x = 18.
Dividing both sides by 1.8, we find that x = 10. Therefore, Monica averaged 10 miles per week last month.