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Comment on Calculating Combinations
God bless u.you soo good
Thank you.
you are great!!! THIS SHORT
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
Thanks for the kind words,
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
Regarding the pizza problem.
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?
Good question!!
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
https://gre.myprepclub.com/forum
Since no toppings were repeated, shouldn't the answer be 6x5x4?
The key here is that the
The key here is that the order in which we select the pancake toppings does not matter.
For example, choosing strawberries, blueberries and whipped cream as pancake toppings is the same as choosing blueberries, whipped cream and strawberries as pancake toppings.
His order and which we select the toppings does not matter, will use combinations.
Hey, can you help me with the
If this were an official GRE
If this were an official GRE question, there would be an extra proviso explaining that arrangements where each person's RELATIVE position is the same are considered identical. Here's what I mean:
Let's say there are 4 people seated at a circular table, and A is seated in the top chair, and in clockwise order, we have B, C and D.
This arrangement is considered the same as one in which B is seated in the top chair, and in clockwise order, we have C, D and A.
In both arrangements, B is to the left of A, C is to the left of B, D is to the left of C, and A is to the left of D. As such, those two arrangements are considered equivalent.
To account for these duplicate arrangements, we have the following general formula:
We can seat n people at a circular table in (n-1)! ways.
So, for this question, we can arrange the 7 people in (7-1)! ways.
(7-1)! = 6! = 720.