The 26 letters of English are given ranks 1,2, ...., 26, in the alphabetical order. Then for $$1 \leq r \leq 26, r^{th}$$ letter is coded as $$k^{th}$$ letter where $$7r + 3 = 26M + k$$ where $$0 \leq k \leq 25$$ and $$M ∈ Z$$. When $$k = O, k^{th}$$ letter is coded as 'z'. For example the letter 'E' is coded as 'L' because when $$r = 5$$ and $$7r + 3 = 38$$, then $$k = 12$$ and $$12^{th}$$ letter is 'L'. For decoding the reverse process is used. Based on this answer the questions.