1.In how many different ways can 5 ones and 20 twos be permuted so that each one is followed by atleast 2 twos?
2.How many numbers from a set {1,2,3…,20} should be chosen in order to be sure to have one number multiple of another.
3.How many numbers must be chosen from a set {1,2,3…8} such that atleast 2 of them must have sum=9.