Change as few characters as possible in the following sentence to make it correct:

The number 99 916 804 is equivalent to 11.

*Note: You cannot add or remove any characters within the sentence.

Change 804 to 806. 916-806-99=11

How about:

The number 99+916-804 is equivalent to211.

Requires changing the first space character in the number to +, the second space to -, and change the space before the 11 to 2.

Pready, that doesn't work very well unless you swap the numbers around. And since you can't say -99+916-806 without losing the space between "number" and "99", I think the better way is to change the 916 to 716. Then you can write "The number 99+716-804 is equivalent to 11." Like your solution, only three characters changed, but there's no rearranging necessary.