Let S=\p1,p2 ,p10\ be the set of first ten prime numbers. Let A=S P, where P is the set of all possible products of distinct elements of S. Then the number of all ordered pairs (x,y), x S, y A, such that x divides y, is .
Answer
5120
Solution
Each y A is a product of a non-empty subset of the 10 primes (different subsets give different numbers).
pi y pi is in the subset; the other 9 primes are free: 29=512 choices
pairs =10 512=5120
Answer: 5120