\documentclass[a4paper]{article}
\usepackage{pgffor,calculator}
\usepackage[box,insidebox]{automultiplechoice}

\def\place{1}
\def\AMCchoiceLabel#1{\SUBTRACT{\arabic{#1}}{\place}{\answer}\answer}

\begin{document}
	\begin{questionmult}{abc123}	
		\begin{choicescustom}[o]
			\def\place{1}\\		% place takes care of choice label: see choiceLabel redefinition above					
			\foreach \n in {1,2,3,...,10}{
				\SUBTRACT{\n}{1}{\myscore}
				\MULTIPLY{100}{\myscore}{\myscore}
				\correctchoice{}\scoring{b=\myscore,m=0}
				}
				\\
			\def\place{11}
			\foreach \n in {1,2,3,...,10}{
				\SUBTRACT{\n}{1}{\myscore}
				\MULTIPLY{10}{\myscore}{\myscore}
				\correctchoice{}\scoring{b=\myscore,m=0}
			}
			\\
			\def\place{21}
			\foreach \n in {1,2,3,...,10}{
				\SUBTRACT{\n}{1}{\myscore}
				\correctchoice{}\scoring{b=\myscore,m=0}
			}\\[15pt]
				\end{choicescustom} 								
  					\end{questionmult}
\end{document}