Solution: The first person has four permutations, the first person has three permutations, and so on. A * * * has 4×3×2× 1=24 permutations.
2. There are 250 books on the upper shelf and 1 10 books on the lower shelf. Now the upper and lower levels have borrowed the same number of books, and the remaining upper level is just three times as much as the lower level. How many books have you borrowed?
Solution: the difference between the upper and lower books is 250-110 =140, which is 3- 1=2 times that of the lower books. So 70 books are lent from the lower level, and the lent books are 1 10-70=40.
3. Simple algorithm: 9+99+999+9999+99999+999999.
Write (10-1)+(100-1)+(1000-1) and the answer is1/.