If the correct answer to a numerical question is non-zero, then that correct answer can be shown when an incorrect submission is made. This is done by selecting 'Answers' in the Review options of the quiz setup. However, if the correct answer is '0' (zero) then the correct answer does not display on submission. It also appears that the number of incorrect attempts on a question where the answer is zero is not being recorded.

This was initially posted in the Quiz forum http://moodle.org/mod/forum/discuss.php?d=141424 and Pierre Pichet replied,

"This is related to the test done for showing or not the Correct answer
like line 306 of shortanswer/questiontype.php

if ($correctanswer) {
echo ('<div class="correctness">');
print_string('correctansweris', 'quiz', s($correctanswer, true));
echo ('</div>');
}
if $correctanswer == 0 , it will not show.
If not already done, I will fill a bug report later this day (with the solution...)"