$\hat{y} = 30$ cm; this is interpolation because $x = 40$ is a reasonable value

$\hat{y} = 30$ cm; we cannot determine if this is extrapolation without knowing the data range

$\hat{y} = 20$ cm; this is extrapolation and the prediction is unreliable

$\hat{y} = 30$ cm; this is always interpolation for positive $x$ values