The heat flow around my portfolio location is 80 mWm^-2 

(ii) As an approximation for most minerals in rocks with thermal conductivity ~3 watts/meter/°C, you can use Fourier's law to convert this heat flow value to a geothermal gradient (in °C/km) simply by dividing your portfolio's milliwatt-per-square-meter heat flow value (from above) by the number 3. According to this simple approximation, what is the present-day geothermal gradient of your portfolio area?