Create a StringLowerUpper Factory

[MathyouMB]

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>