You are right. Without knowing the value of n or without any other information available, it is not possible to determine the value of 5n +1. This could be the term of an expression where 5n +1 represents an integer that is 1 more than a multiple of 5.
From 5n+1, it can only be inferred that the number is 1 more than a multiple of 5.
The ques 5n+1 = is in complete, because it is not given that
5n + 1 equal to how much. If we take 5n+ = any number say (x)
then we can say 5n = 1 less than the number x. i.e. 5n = x -1
Or n = (x -1) / 5.
For example If we say 5n + 1 = 11 then 5n = 11-1 =10
i.e. 5n = 10 Or n = 10/2 =5
hence n = 5 will be the answer