A set consists of five different odd positive integers, each greater than 2. When these five integers are multiplied together, their product is a five-digit integer of the form , where and are digits with and . (The hundreds digit of the product is zero.) For example, the integers in the set have a product of 45045. In total, how many different sets of five different odd positive integers have these properties?
Solution
Solution 1: Let and let be the two-digit integer . We note that , and that . Therefore, . We want to write as the product of 5 distinct odd integers, each greater than 2, and to count the number of sets of such odd integers whose product is . There are several situations to consider. First, we look at possible sets that include the integers 7, 11 and 13 (which we know are divisors). Second, we look at possible sets that include two of these integers and an odd multiple of the third. Third, we rule out possible sets that include one of these integers and odd multiples of the second and third. Fourth, we rule out possible sets that include the product of two or three of these integers and additional integers. Case 1: where and Here, and so . This tells us that is less than 100. If , then the possible values for are . These give the following corresponding values of : . Note that , since and gives which has two equal digits and so is not possible. If , then the possible values for are . If , then since the integers in are odd and distinct, and so , which is not possible. Therefore, in this case, there are 15 possible sets. Case 2: where and is odd and Here, we have and so which gives . Note that . Suppose that . This means that . If , then the possible values of are 5 and 9 since and are odd, greater than 2, and distinct. ( is not possible, since this would give the set which is already counted in Case 1 above.) If , then which gives , which is not possible. Suppose that . This means that . If , then . There are no further possibilities when . Since and , then we cannot have . Therefore, in this case, there are 3 possible sets. Case 3: where and is odd and Suppose that . This means that . If , then the possible values of are 5 and 9. (Note that .) We cannot have as this would give and a product of 99099 which has equal digits and . We cannot have since this gives . Suppose that . This means that . If , then . As in Case 2, we cannot have . Therefore, in this case, there are 3 possible sets. Case 4: where and is odd and Suppose that . This means that . If , the possible values of are 5 and 9. (Again, in this case.) We cannot have when otherwise . If , we can have and but there are no other possibilities. As in Cases 2 and 3, we cannot have . Therefore, in this case, there are 3 possible sets. Case 5: where and are odd and Here, . Since are odd, then which means that . Since there do not exist two distinct odd integers greater than 1 with a product less than 15, there are no possible sets in this case. A similar argument rules out the products , , where are odd integers greater than 1. Case 6: where and Note that since we know that has divisors of 7 and 11. Here, . Since , there are no possible sets in this case, nor using or in the product or 1001 by itself or multiples of 77,91 or 143. Having considered all cases, there are possible sets. Solution 2: We note first that , and that . Therefore, . Since is odd, then is odd. Since and and is odd, then we have the following possibilities for the two-digit integer : . If the integer is a prime number, then cannot be written as the product of five different positive integers each greater than 2, since it would have at most four prime factors. Using this information, we can eliminate many possibilities for from our list to obtain the shorter list: . Several of the integers in this shorter list are the product of two distinct prime numbers neither of which is equal to 7,11 or 13. These integers are and and and and and and and . Thinking about each of these as for some distinct prime numbers and , we have . To write as the product of five different positive odd integers greater each greater than 2, these five integers must be the five prime factors. For each of these 8 integers , there is 1 set of five distinct odd integers, since the order of the integers does not matter. This is 8 sets so far. This leaves the integers . Seven of these remaining integers are equal to the product of two prime numbers, which are either equal primes or at least one of which is equal to 7,11 or 13. These products are and and and and and and . In each case, can then be written as a product of 5 prime numbers, at least 2 of which are the same. These 5 prime numbers cannot be grouped to obtain five different odd integers, each larger than 1, since the 5 prime numbers include duplicates and if two of the primes are combined, we must include 1 in the set. Consider, for example, . Here, . There is no way to group these prime factors to obtain five different odd integers, each larger than 1. Similarly, and . The remaining three possibilities (35, 49 and 65) give similar situations. This leaves the integers to consider. Consider . There are 6 prime factors to distribute among the five odd integers that form the product. Since there cannot be two 3's in the set, the only way to do this so that they are all different is . Consider . There are 7 prime factors to distribute among the five odd integers that form the product. Since there cannot be two 3 s or two 9 s in the set and there must be two powers of 3 in the set, there are four possibilities for the set : . Consider . There are 6 prime factors to distribute among the five odd integers that form the product. Since two of these prime factors are 3, they cannot each be an individual element of the set and so one of the 3 s must always be combined with another prime giving the following possibilities: . Consider . Using a similar argument to that in the case of 45045, we obtain . Finally, consider . There are 6 prime factors to distribute among the five odd integers that form the product. Since we cannot have two 3 s or two 7 s in the product, the second 3 and the second 7 must be combined, and so there is only one set in this case, namely . We have determined that the total number of sets is thus .