First, recall that the tex is a unit of mass per unit length of a fibre:
\[
\text{Tex} = \frac{\text{Mass (in mg)}}{\text{Length (in meters)}}.
\]
We are given:
\[
\text{Tex} = 0.16 \, \text{tex}, \text{Length} = 40 \, \text{mm} = 0.04 \, \text{m}, \text{Mass} = 24 \, \text{mg}.
\]
To find the number of fibres \( N \), use the formula:
\[
N = \frac{\text{Total Mass}}{\text{Mass per Fibre}} = \frac{24 \, \text{mg}}{0.16 \, \text{tex} \times 0.04 \, \text{m}} = 3750.
\]
Thus, the number of fibres is \( 3745 \) fibres.