Number---^1---^2---^3---^4------Cyclicity
2----------- 2-----4------8-----6----------4
3----------- 3-----9------7-----1----------4
4----------- 4-----6------4-----6----------2
5----------- 5-----5------5-----5----------1
6----------- 6-----6------6-----6----------1
7----------- 7-----9------3-----1----------4
8----------- 8-----4------2-----6----------4
9----------- 9-----1------9-----1----------2The table above gives the cyclicity of any number.
If needed, memorize this table for numbers
2,3,7,8 which have the cyclicity of 4.
However, if we need to derive the cyclicity for number 9 and 7
we can do so by this methodFor this problem
Units digit
\(9^odd\) = 9
\(9^even\) = 1
\(7^1\) = 7
\(7^2\) = 9
\(7^3\) = 3
\(7^4\) = 1
From the above table, we can clearly say that \(9^19 - 7^15\).
Since 9^19 has units digit 9 and 7^15 has units digit 3,
we will have units digit for the expression as 9-3(6)
(Option D)
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79% (01:17) correct 
