1. **Problem Statement:** A technician is testing the exhaust emissions of a car running smoothly but with poor gas mileage. The technician uses two dials: one for Hydrocarbon (HC) levels from 0 to 2000 ppm and one for Carbon Monoxide (CO) levels from 0 to 10%. The black button indicates which scale to read. 2. **Understanding the Problem:** The technician needs to interpret the readings on the dials correctly based on the black button's position. The HC dial measures parts per million (ppm) of hydrocarbons, and the CO dial measures percentage of carbon monoxide. 3. **Formula and Rules:** - Hydrocarbon concentration is read directly from the HC dial scale (0 to 2000 ppm). - Carbon Monoxide concentration is read from the CO dial scale (0 to 10%). - The black button selects which dial's scale is active for reading. 4. **Interpreting the Readings:** - If the black button is set to HC, read the value on the HC dial. - If the black button is set to CO, read the value on the CO dial. 5. **Example:** - Suppose the black button is on HC and the needle points to 1200 on the HC dial, then HC concentration is 1200 ppm. - If the black button is on CO and the needle points to 1.5 on the CO dial, then CO concentration is 1.5%. 6. **Conclusion:** The technician must ensure the black button is set correctly to read the desired gas concentration accurately. This helps diagnose the car's emission performance and relate it to gas mileage issues. No complex algebraic calculations are needed here; the key is understanding the scale selection and reading the correct dial.