It is clear that the relative velocity will only be perpendicular to the axis

1 / v + 1 / u = 1 / f

so,

v = uf / u+f

I = O v / u

I = (0.5 cm) f / (f + u)

dI/dt = -(0.5cm) f / (f + u)^2 * du/dt

Putting f = 40 cm and u = -30 cm, du/dt = 4cm/s,

|dI/dt| = 0.8 cm/s = v

This is the speed with which one image will approach the principal axis. However, since we want relative velocity, we need the double of v which is 1.6 cm/s