The speed of the snail is given by:
[tex] v=\frac{S}{t} [/tex]
where S is the distance covered by the snail and t the time taken.
The snail in the problem moves by S=18 mm in t=24 s, therefore its speed is
[tex] v=\frac{S}{t}=\frac{18 mm}{24 s}=0.75 mm/s [/tex]
