swerve wrote: ↑

Sat Feb 16, 2019 9:56 am

Flor is choosing three of five colors of paint to use for her art project at school. Two of the colors, Green and Yellow, cannot both be selected. How many different ways can Flor choose the colors for her project?

A. 7
B. 9
C. 10
D. 13
E. 17

The OA is A

Source: Princeton Review

Mitch's approach is the approach that I'd typically use. However, it's important to note that, when the answer choices are so small (as they are here), we should also consider the straightforward strategy of listing and counting

Let R, B, P, G and Y represent the colors Red, Blue, Purple, Green and Yellow respectively.

Now let's list the possible outcomes that meet all of the given conditions:
- RBP
- RBG
- RBY
- RPG
- RPY
- BPG
- BPY
Done!! So, there are 7 possible outcomes

Answer: A

Cheers,
Brent

swerve wrote: ↑

Sat Feb 16, 2019 9:56 am

Flor is choosing three of five colors of paint to use for her art project at school. Two of the colors, Green and Yellow, cannot both be selected. How many different ways can Flor choose the colors for her project?

A. 7
B. 9
C. 10
D. 13
E. 17

The OA is A

Source: Princeton Review

Solution:

There are three cases: 1) green is one of the five colors chosen, but yellow isn’t, 2) yellow is one of the five colors chosen, but green isn’t, and 3) neither green nor yellow is chosen. Let’s analyze each case.

Case 1: Green is one of the five colors chosen, but yellow isn’t.

If green is chosen but yellow isn’t, then we have to choose 2 more colors from the 3 remaining colors. The number of ways to do that is 3C2 = 3.

Case 2: Yellow is one of the five colors chosen, but green isn’t.
This is analogous to case 1, so there are 3 ways for this case.

Case 3: Neither green nor yellow is chosen.

If neither color is chosen, then we have to choose 3 colors from the 3 remaining colors. The number of ways to do that is 3C3 = 1.

Thus, the total number of ways Flor can choose the colors for her project is 3 + 3 + 1 = 7.

Alternate Solution:

We can use the formula:

Number of ways where yellow and green are not both included = total number of ways to pick 3 colors - number of ways where yellow and green are both included
Since we are choosing 3 colors from 5 available colors, there are 5C3 = (5 x 4)/2 = 10 ways of doing this when there are no restrictions.

The number of ways in which yellow and green are both included can be found easily by observing that yellow and green occupy two of the three slots; any one of the remaining three colors can occupy the final slot. So, there are 3 ways to choose colors where yellow and green are both included.

Thus, the number of ways to pick colors where yellow and green are not included together is 10 - 3 = 7.

Answer: A