Answer
$$1.55 \ rad .$$
Work Step by Step
Since the distance that the wheel travels is given by $s=r\theta$, then by substitution and using the fact that the radius is half of the diameter, we have
$$ 20= (25.8/2) \times \theta \Longrightarrow 2\times20= 25.8\times \theta \\\Longrightarrow\theta=\frac{2\times 20}{25.8}=1.55 \ rad .$$
