Subject: Functions Name: Rachel Who is asking: Student Level: Secondary I need help with these two problems! The teacher didn't explain clearly on how to do it. I'm lost! Please help. Thanks How many different numbers of 2 different digits each can be formed using the digits (1,3,5,7,9)? How many triangles are formed by the vertical of a regular hexagon? Hi Rachel, Imagine that you are goint to list all such 2 digit numbers and, at each stage, count the number of choices you have. To write a 2 digit number you start with the first digit. For this digit you have 5 choices, either 1, 3, 5, 7 or 9. Thus you list so far is 1,     3,     5,     7,     9,     Choose one of the items on you list, say the one that starts with 7. If you want to extend this to a 2 digit number you have 4 choices for the second digit, 1, 3, 5 or 9. Thus the possible 2 digit numbers that start with 7 are 7, 1 7, 3 7, 5 7, 9 What is important to see here is that whichever 1 digit number you chose from the first list, it expands to 4, 2 digit numbers. Thus it total you have 5 x 4 = 20 possible 2 digit numbers. Now can you solve the second problem? Penny Go to Math Central