can someone explain matthew chen solution
see link below
https://usaco.guide/problems/acMultipleOf2019/usersolutions
questions about matthew chen comments
 i don’t understand what this number means
d_k + 10d_{k+1} + 100d_{k+2} + … + 10^(nk)d_n  why do we multiple by 10^(k1)?
 and how does multiplying number above by 10^(k1) turns into this?
S_n  S_{k1} = 0 (mod 2019) <=> S_n = S_{k1} (mod 2019)  also, what does S_{k1} mean?
 how is this true? 10^(1) = 202 (mod 2019)
isn’t this 0.1 = 202 (mod 2019)? not sure how they got 202  why do we do this " multiply subsequent terms by a factor of 202 to form an equivalent sequence for S_n in the field F_{2019}."
 can someone give a better summary than mines?

 take mod of substring

 multiply remainder from 1. by factor than take mod again
 Q: why do multiply by factor, isn’t that redundant?
 why not just use the remainder directly as a hash key?
 3.use remainder from 2. as a key for hash residues

 calculate answer from hash residues