Assumption: The unit "ms⁻²" for velocity is a typo and should be "ms⁻¹". We will proceed with velocities u1 = 70 ms⁻¹ and u2 = 55 ms⁻¹.

Here's how to solve the problem using the principle of conservation of momentum.

Assumption: The unit "ms⁻²" for velocity is a typo and should be "ms⁻¹". We will proceed with velocities $u_1 = 70 \text{ ms}^{-1}$ and $u_2 = 55 \text{ ms}^{-1}$.

The principle of conservation of momentum states that for a collision where bodies stick together, the total momentum before the collision equals the total momentum after the collision.
The formula is:

$$m_1 u_1 + m_2 u_2 = (m_1 + m_2) V$$

where $m_1$ and $m_2$ are the masses, $u_1$ and $u_2$ are their initial velocities, and $V$ is their common final velocity.

Given:
$m_1 = 30 \text{ kg}$
$u_1 = 70 \text{ ms}^{-1}$
$m_2 = 40 \text{ kg}$
$u_2 = 55 \text{ ms}^{-1}$