Let's use some facts ('a mod z' means remainder of division of a by z):
1) 10 mod 11=−12) 100 mod 11=1
Replacing terms with their remainders of division will not affect on the resulting remainder:
3) (a+b) mod z=(a mod z+b) mod z=(a mod z+b mod z) mod z4) (ab) mod z=(a mod z⋅b) mod z==(a mod z⋅b mod z) mod z5) (xp) mod z=(x mod z)p mod z
Let ai be digits of number n
n is divisible by 11 if and only if n mod 11 = 0
n mod 11=(a0+10a1+102a2+...+10kak) mod 11==(a0+102a2+...+102pa2p++10a1+103a3+...+102q+1a2q+1) mod 11=(a0+...+(100 mod 11)pa2p+(10 mod 11)a1+...+(10 mod 11)2q+1a2q+1) mod 11=(a0+...+a2p−a1−...−a2q+1) mod 11n mod 11=0⇔⇔(a0+...+a2p−(a1+...+a2q+1)) mod 11=0
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