(a) The change in gravitational potential energy (PE) is given by:

\(\text{PE} = mgh\)

where \(m = 5 \text{ kg}\), \(g = 9.8 \text{ m/s}^2\), \(h = s \sin \theta\), and \(s = vt = 1.8 \times 15 \text{ m}\).

\(h = 1.8 \times 15 \times \sin 20^{\circ}\)

\(\text{PE} = 5 \times 9.8 \times 1.8 \times 15 \times \sin 20^{\circ}\)

\(\text{PE} = 462 \text{ J}\)

(b) The work done (WD) by the pulling force is the sum of the work done against gravity and the work done against friction:

\(\text{WD} = \text{PE} + F_f \times s\)

\(\text{WD} = 5 \times 9.8 \times 1.8 \times 15 \times \sin 20^{\circ} + 40 \times 1.8 \times 15\)

\(\text{WD} = 1540 \text{ J}\)