4 The mass, \(m\) grams, of a substance is increasing exponentially so that the mass at time \(t\) hours is given by
$$m = 250 \mathrm { e } ^ { 0.021 t } .$$
- Find the time taken for the mass to increase to twice its initial value, and deduce the time taken for the mass to increase to 8 times its initial value.
- Find the rate at which the mass is increasing at the instant when the mass is 400 grams.