Lesson: Calculating Combinations

Comment on Calculating Combinations

God bless u.you soo good

Thank you.

you are great!!! THIS SHORT CUT IS IS NOT THOUGHT IN SCHOOLS.

thank you, YOU ARE BEST MATH TEACHER.

-> I HAVE ONE MORE QUESTION -> IF I HAVE DONE A WRONG STEP CALCULATION NPR INSTEAD OF NCR FOR "ORDER DOESNT MATTER" SELECTION QUESTIONS, CAN i STILL GET THE CORRECT ANSWER BY DIVIDING THE OUTCOME WITH 2.

-> IN SHORT WHAT IS MEANT IS => COMBINATION=1/2(OUTCOME OF PERMUTATION) AT LEAST WHEN THERE IS A TOTAL VALUE N A EVEN NUMBER. HOW ABOUT THE APPROACH? ALWAYS DIVIDE OUTCOME BY 2 AND RECHECK WITH ANSWER CHOICES? WILL THIS WORK?
PLEASE RPLY -THANKS AND REGARDS VINEET
greenlight-admin's picture

Thanks for the kind words, Vineet.

In general, if you accidentally use nPr (instead of nCr), you can find the correct result if you divide your calculation by r!

So, for example, if you accidentally calculate 5P2 (instead of 5C2), you can find the correct result if you divide your calculation by 2!

Does that help?

Cheers,
Brent

ASIDE: I'm not a big fan of permutations as they pertain to GRE counting questions. For more on this, read: https://www.greenlighttestprep.com/articles/combinations-and-non-combina...

Great tip. This is exact what I was looking for. I was spending way too much time on calculations.

Regarding the pizza problem. How can I tell that it's not a combination with repetition problem? On the mathisfun site, their is an example problem with ice-cream which states: "Let us say there are five flavors of icecream: banana, chocolate, lemon, strawberry and vanilla.

We can have three scoops. How many variations will there be?"

I don't see how the ice cream problem is different from the pizza problem, but it must be.

Oh, I guess it's assumed we can not repeat the pizza topping? So, that would make it a combination without repetition problem?
greenlight-admin's picture

Good question!!

In your ice cream example, there's no text that says we can't have 2 or more scoops of the same ice cream.

However, the pizza question uses the term "3-topping pizza," which (for me) suggests the 3 toppings must be different.
For example, if we chose mushrooms, mushrooms, and mushrooms for the pizza, is it still a 3-topping pizza?
That said, I fully recognize that I may have chosen an ambiguous way to phrase the question.
It would probably be better if I had added a proviso that says "the 3 toppings must be different"

Cheers,
Brent

Add a comment

Have a question about this video?

Post your question in the Comment section below, and I’ll answer it as fast as humanly possible.

Change Playback Speed

You have the option of watching the videos at various speeds (25% faster, 50% faster, etc). To change the playback speed, click the settings icon on the right side of the video status bar.

Let me Know

Have a suggestion to make the course even better? Email me today!

Free “Question of the Day” emails!