Find the units digit of the sum of the first $21$ positive perfect cubes.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Find the units digit of the sum of the first $21$ positive perfect cubes.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
1
Find the units digit of the sum of the first $21$ positive perfect cubes.
train