5 A substance is decaying in such a way that its mass, m kg , at a time t years from now is given by the formula
$$\mathrm { m } = 240 \mathrm { e } ^ { - 0.04 \mathrm { t } }$$
- Find the time taken for the substance to halve its mass.
- Find the value of \(t\) for which the mass is decreasing at a rate of 2.1 kg per year.