All of the digits you've listed are significant. If we're supposed to perform some operation on them, the least number of significant figures in any number of the problem determines the number of significant figures in the answer.
3 significant figures, the 256
256 (162)
256 numbers.
256
256 unless you count zero in which case its 625.
Three, a 2, a 5 and a 6.
256
Without repeating any of the digits: 4*3*2*1 = 24 If repeated digits are allowed, then 4*4*4*4 = 256
It depends on the encryption strength. For 64 bit connections, the key is 10 digits long. For 128 bit connections, the key is 26 digits long. For 256 bit connections, the key is 58 digits long.
256
How about 256
256