Only modifying a certain digit of 21475 can make the modified number divisible by 15. How should it be modified? Write all the modified plans below, and briefly explain the basis for the modification.
只修改21475的某一位数字，就可以使修改后的数能被15整除．应怎样修改？把所有修改的方案写在下面，并简要说明修改根据．
<result>3</result>