Question: 3-Digit Odd Numbers

Comment on 3-Digit Odd Numbers

Shouldn't the 1st digit be able to completed in 4 ways ?
greenlight-admin's picture

In the solution, we selected the units digit first. This can be accomplished in 2 ways.
My second task was to select a hundreds digit. Since I had already selected ONE digit for the units digit (and since repeated digits are not allowed), there were only 5 digits to choose from (the original 6 digits MINUS the 1 digit I selected to be the units digit)
Does that help?

I am generally confused when to use counting and when to using combination. In this case it seemed like 2* 2C5. How do i distinguish between these two things ?
greenlight-admin's picture

In the MAJORITY of counting questions on the GRE, the Fundamental Counting Principle is all you need. However, there ARE times when you'll need to use some combinations. The following video addresses that very issue:

Fayes question is similar to mine. After choosing ways in which to accomplish the restrictive stage, the remaining unused digits are 4. However u say it's 5 ways to accomplish 1st stage. This becomes confusing at that stage
greenlight-admin's picture

Our first step is to choose a digit (a SINGLE digit) to go in the units place. There are two ways to accomplish this step (we can choose EITHER 1 OR 5)

Once we have placed a SINGLE digit in the units position, there are 5 digits remaining. So, we can complete the next step in 5 ways.

Does that help?


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