The work done \((WD)\) by the force is given by the formula:

\(WD = Fd \cos \theta\)

where \(F = 30 \text{ N}\) is the force, \(d\) is the distance moved, and \(\theta\) is the angle of the force above the horizontal.

Since the block moves at a constant speed of 1.5 m/s for 20 seconds, the distance \(d\) is:

\(d = 1.5 \times 20 = 30 \text{ m}\)

Substituting the given values into the work done formula:

\(720 = 30 \times 30 \cos \theta\)

Simplifying gives:

\(720 = 900 \cos \theta\)

\(\cos \theta = \frac{720}{900} = 0.8\)

Therefore, \(\theta = \cos^{-1}(0.8) \approx 36.9^\circ\).