While racing around the track last week, Marco was able to get to 120 mph. This week Marco tried again and was able to reach 135 mph. What was the percentage increase from last week to this week?

To find the percentage increase, we need to calculate the difference between Marco's speeds and divide it by his speed last week. The difference is 135 mph - 120 mph = <<135-120=15>>15 mph.

The percentage increase is (15 mph / 120 mph) × 100% = 12.5%. Answer: \boxed{12.5}.