In the given graph we can observe the gasoline mileage (mpg) for two different cars at different constant speeds (mph).
We need to determine at what 10 mph increment (x-axis) the driver would first notice a decrease in gasoline mileage.
As can be observed, the full-size car (red line) gasoline mileage remains constant at 10 mph and 20 mph, but the compact car (blue dashed line) shows a decrease in the gasoline mileage when the mph increments from 10 mph to 20 mph.
At 10 mph, the mpg is 25, and at 20 mph, the mpg is 24.
The driver would first notice a decrease in gasoline mileage with the compact car when the speed goes from 10 mph to 20 mph.