5 The mass, \(M\) grams, of a certain substance is increasing exponentially so that, at time \(t\) hours, the mass is given by
$$M = 40 \mathrm { e } ^ { k t }$$
where \(k\) is a constant. The following table shows certain values of \(t\) and \(M\).
- In either order,
(a) find the values missing from the table,
(b) determine the value of \(k\). - Find the rate at which the mass is increasing when \(t = 21\).