Particles of coal about 50 micrometers (m) in size are injected into a stream of air at

Particles of coal about 50 micrometers (μm) in size are injected into a stream of air at 300°C that carries them to a boiler. How long will it take to heat the particles from 25°C to within 5°C of the air temperature? Assume that the external coefficient h is given by hD_p/k = 2.0, where D_p is the particle diameter.