11. (10 points) If 2 tennis rackets and 7 tennis balls cost a total of 220 yuan, and 1 tennis racket is 83 yuan more expensive than 1 tennis ball. Find the unit price of the tennis ball.
【Solution】Solve: $220-83 \times 2$
$$
\begin{array}{l}
=220-166 \\
=54 \ (\text{yuan}) \\
54 \div (2+7) \\
=54 \div 9 \\
=6 \ (\text{yuan})
\end{array}
$$

Answer: Each tennis ball costs 6 yuan.