(i) Using Newton's second law, the net force on the car is given by:

\(F - 900 = 1200a\)

where \(F\) is the force provided by the engine, and \(a\) is the acceleration (or deceleration in this case).

The power \(P\) of the engine is related to the force and speed by:

\(F = \frac{P}{v}\)

Substituting the given values:

\(F = \frac{18000}{25} = 720 \text{ N}\)

Substitute \(F\) back into the equation:

\(720 - 900 = 1200a\)

\(-180 = 1200a\)

\(a = -0.15 \text{ m s}^{-2}\)

Thus, the deceleration is 0.15 m s^-2.

(ii) To find the least speed, set the net force to zero:

\(\frac{18000}{v} - 900 = 0\)

Solve for \(v\):

\(\frac{18000}{v} = 900\)

\(v = \frac{18000}{900} = 20 \text{ m s}^{-1}\)

Therefore, the least speed is 20 m s^-1.