## Link of the Day

You shouldn't judge a book by it's cover but, many people do so anyway. In order to interest teenagers in classics books like "Emma" and "Wuthering Heights", publishers are updating the covers with modern artwork so that they resemble modern books like "Twilight" or "The Hunger Games". You can read a great article about these new book covers here. This story would make a great current event "Excellent Example" for your essay.

## 6/29 Combinations

*Read the Following SAT Question and then select the correct answer.*

*As always, remember to follow the Knowsys method for math. Read the problem carefully and identify the bottom line (what you are looking for). Then, consider your options. How could you solve it? How should you solve it? There are usually several different ways to solve a problem. You should choose the method that gets you to the correct answer as quickly and reliably as possible. Next, attack the problem using the method that you have selected. Finally, loop back and verify that your answer matches the bottom line.*

*Cathy likes to wear 2 ribbons, each a different color. If she has 6 ribbons each of a different color, how many combinations of 2 different-colored ribbons can she select to wear?*

**combination**problem. If you have learned how to work

**combination**problems it is very straightforward. You know that Cathy is selecting 2 ribbons out of 6 ribbons. When she selects the first ribbon, she has 6 different ribbons to choose from. When she selects the second ribbon, she has 5 ribbons to select from (since 1 of the ribbons has already been chosen). If this were a

**permutation**problem, (in other words, if we cared if she chose a red ribbon first and then a blue ribbon or if she chose a blue ribbon first and then a red ribbon) then you would just multiply 6 and 5 and the correct answer would be 30. However, since you only care about which 2 ribbons she chooses (and not the order she chooses them in) you need to eliminate all the duplicates. There will be half as many different

**combinations**as there are

**permutations**so the final step is to divide 30 by 2. There are 15 different

**combinations**of ribbons that Cathy can wear.

(

**A**) 3

(

**B**) 6

(

**C**) 12

(

**D**) 15

(

**E**) 36

The correct answer choices is (

**D**)

On sat.collegeboard.com 44% of the responses were correct.

For more help with math visit www.myknowsys.com.