Why the query optimizer choose this bad execution plan?

$$ 82=\text{mean score}=\frac{\text{sum of all scores}}{30}=\frac{\text{missing score}+\text{sum of all others}}{30}. $$ Therefore $$ 30\cdot82 = \text{missing score}+\text{sum of all others}. $$ $$ 30\cdot82=\text{missing score}+\left(29\cdot\text{mean of all others}\right)= \text{missing score}+(29\cdot84). $$ So $$ 30\cdot82=\text{missing score}+(29\cdot84). $$ Can you find the missing score given that?

Not introducing a spurious space before the url in the first argument to \href seems to solve the problem. I say 'seems' because I do not have access to that version of Reader or to Windows. Hence, I tested this with an older version on GNU/Linux. However, I reproduced the problem and tested the solution, so I'm guessing it is the same. (If not, let me know and I'll delete this answer.)

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\href{http://www.kleptocrat.edu}{Prof.Gas Bag}

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