回复：5th grader math rocks! problem - r u smarter than a 5th grade

S=3*13(1+3^3+3^6+3^9+.....+3^57)
=3*5*2*2*2(1+3^4+3^8+3^12+....+3^56)
=3*11*11(1+3^5+3^10+......+3^55)
So,S is divisible by 2x2x2x3x5x11x11x13.
Ha,I can graduate from elementary school.