Sum of Digits base 10 | PRMO II 2019 | Question 7

Let s(n) denote the sum of the digits of a positive integer n in base 10. If s(m)=20 and s(33m)=120, what is the value of s(3m)?

taking sum of digit base 10 to (mod 9)

and s(ab)=s(a).s(b)(mod 9)

[ let x congruent r mod n, y congruent to s mod n,

\(0 \leq r,s \leq n-1\),

x=in+r, y=jn+s, i,j are integers

xy=(in+r)(jn+s)=ij\(n^2\)+(is+jr)n+rs congruent to rs mod n

so, xy mod n =(x mod n)(y mod n) ]

given s(m)=20

s(33m)=120=\(s(11) \times s(3m)\)

or, 120=\(2 \times s(3m)\) [ since s(11)=2(mod 9)]

or, 60=s(3m)

so, s(3m)=60.

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