For this generator we can randomize the length and the number of upper case and lower case letters we care about
<multiple-choice>
<question>
<![CDATA[
Consider strings of length 15, where each character is a lowercase letter or an uppercase letter.
How many such strings contain at least one lowercase letter and at least one uppercase letter?
]]>
</question>
<answer>$52^{15} - 26^{15}$</answer>
<answer correct="correct">$52^{15} - 2 \cdot 26^{15}$</answer>
<answer>$52^{15} - 3 \cdot 26^{15}$</answer>
<answer>None of the above.</answer>
</multiple-choice>
For this generator we can randomize the length and the number of upper case and lower case letters we care about