Question 13
The paper defines a special power notation $n!!$ by the following tower and example. Find the last digit of $2021!!-2020!!$.
Notation note: this is the paper's special notation, not the usual double factorial. The printed tower and its parenthesised example use inconsistent grouping. For this question, either grouping makes each expression a positive integer power of its first number, so the requested last digit is unaffected.
Notation note: this is the paper's special notation, not the usual double factorial. The printed tower and its parenthesised example use inconsistent grouping. For this question, either grouping makes each expression a positive integer power of its first number, so the requested last digit is unaffected.