20. 60 people participate in a riddle answering game, with a total of 10 questions. Each person answers each question once, and they collectively answer correctly 452 times. It is known that everyone answers at least 6 questions correctly, and 21 people answer exactly 6 questions correctly, 12 people answer exactly 8 questions correctly, and the number of people who answer exactly 7 questions correctly and the number of people who answer exactly 9 questions correctly are the same. How many people answered all 10 questions correctly? $\qquad$ people.
【Analysis】Since the number of people who answered 7 questions correctly is equal to the number of people who answered only 9 questions correctly, we can consider all these people as having answered 8 questions correctly. Therefore, the total number of people who answered 8 and 10 questions correctly is: $60-21=39$ people, and the total number of correct answers for those who answered 8 and 10 questions correctly is: $452-6 \times 21=326$ times. Assuming these 39 people all answered 8 questions correctly, then $39 \times 8=312$ times, $326-$ $312=14$ (times), $14 \div(10-8)=7$ people. Based on this, we can solve the problem.

【Solution】Solution: Since the number of people who answered 7 questions correctly is equal to the number of people who answered only 9 questions correctly, we can consider all these people as having answered 8 questions correctly,

Therefore, the total number of people who answered 8 and 10 questions correctly is:
$$
60-21=39 \text { (people) }
$$

The total number of correct answers for those who answered 8 and 10 questions correctly is:
$$
452-6 \times 21=326 \text { (times), }
$$

Assuming these 39 people all answered 8 questions correctly, then
$$
\begin{array}{l}
39 \times 8=312 \text { (times) } \\
326-312=14 \text { (times) } \\
14 \div(10-8) \\
=14 \div 2 \\
=7 \text { (people) }
\end{array}
$$

Answer: Therefore, the number of people who answered all 10 questions correctly is 7.
The answer is: 7.