



 
Hi Cheri, I don't know where 78 might have come from. If no digit can be repeated, then the first number can be any of the four, the second number any of the remaining three, and the third number any of the remaining two. That means that, in this case, number of combinations is 4x3x2 = 24. If digits can be repeated then a similar calculation shows that the number is 4x4x4=64. Other cases (e.g. the first digit can't be zero and no two consecutive digits can be the same) can be analyzed similarly. Victoria  


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