Final answer:
The temperature at 5 km deep in the Earth's crust, considering an average geothermal gradient of 25 °C/km, would be around 140 °C provided the surface temperature is approximately 15 °C.
Step-by-step explanation:
Considering a typical, average geothermal gradient of about 25-30 °C/km, and applying the lower end of this range for a conservative estimate, the temperature at 5 km deep in the crust would increase by approximately 125 °C from the surface temperature.
Assuming a surface temperature of about 15 °C (59°F), which is close to the average surface temperature of Earth, the temperature at a depth of 5 km would be approximately 140 °C (284°F).