Call a positive integer N a 7-10 double if the digits of the base-7 representation of N form a base

题目

Call a positive integer N a 7-10 double if the digits of the base-7 representation of N form a base

答案

你看那个例子 ,题目中说51,在10进制下是51,在7进制下是102,正是51的二倍,所以才叫7-10.这样就好理解啦