Modular arithmetic - finding a remainder using the cyclicity of powers.
Powers of 3 modulo 5 cycle with period 4: 3^1=3, 3^2=9=4(mod5), 3^3=27=2(mod5), 3^4=81=1(mod5), then it repeats. Since 256 is exactly divisible by 4 (256=4x64), 3^256...
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