1. A numbergreater than 100 is formed with the digits 2,7,7,8. A certain digit cannot appear more than once. How many such numbers can be formed? 2. 4 ballsare put into 2 out of 6 boxes. In how many ways can the balls be put if eachbox only holds 2 balls?
刚刚再记得3条 3. 说x,y,z三个点在数轴上,问y在不在x和z的中间。 1)y=the greatest of the three figures: min(x,z), min(x,y), and min(y,z) 2)y=the least of the three figures: max(x,z), max(x,y), and max(y,z)
4. 说k=1*2*...*25,问k/10^n是不是整数。 1)(n-6)(n-12)=0 2)n<6 把10变(2*5)^n, k 只可以被5个"5" 和12个"2"整除, 条件一知道不是整数, 所以A