Compare the output ratio to the input ratio at each doubling to classify the returns to scale.

(a) Ratio of output $=2=$ ratio of inputs $\Rightarrow$ Constant Returns to Scale. (b) Ratio of output $=2.2>2=$ ratio of inputs $\Rightarrow$ Increasing Returns to Scale.

1. From (1,1) to (2,2): inputs increase by a factor of $\dfrac{2}{1}=2$. Output increases by a factor of $\dfrac{20}{10}=2$. Since the two ratios are EQUAL, output has grown in EXACTLY the same proportion as the inputs — this is Constant Returns to Scale.
2. From (2,2) to (4,4): inputs increase by a factor of $\dfrac{4}{2}=2$. Output increases by a factor of $\dfrac{44}{20}=2.2$. Since $2.2>2$, output has grown in a GREATER proportion than the inputs — this is Increasing Returns to Scale. This firm therefore experiences Constant Returns to Scale at first, and then moves into Increasing Returns to Scale as it doubles its scale again — the reverse order from a firm that starts small and eventually runs into diseconomies, illustrating that there is no single fixed sequence a firm must follow; it depends on the specific technology and scale involved.
   ✓Final answer

   (a) Constant Returns to Scale (output ratio $2=$ input ratio $2$). (b) Increasing Returns to Scale (output ratio $2.2>$ input ratio $2$).

Comparing the THIRD row directly back to the FIRST row (a factor-of-4 input increase against a $44/10=4.4$ output increase) instead of comparing each row to the one immediately before it would still happen to suggest increasing returns here, but it answers a different question than the one asked (returns to scale AT EACH successive doubling) and would give the wrong classification for part (a).