MathAlready the calculator for everyone

Wind chill is a measure of how cold people feel when exposed to wind outside. A good estimate for wind chill can be found using this calculation
wind chill = air temperature - 0.7* (wind speed),
where temperature is measured in degrees Fahrenheit (F) and the wind speed is measured in miles per hour (mph). Suppose the air temperature is 36F and the wind speed is mph. Which of the following is closest to the approximate wind chill?