
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Calculate the coefficient of $x^{21}$ in the expansion of
\[
\left(1 + x + x^2 + \cdots + x^{20}\right)\left(1 + x + x^2 + \cdots + x^{15}\right)^2.
\]
A) 40
B) 51
C) 60
D) 76
E) 90

Present the answer in LaTex format: \boxed{Your answer}
user