Help with QR section!! Please help!

[missdiana]

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Hello
I am solving Barron's DAT Model Exam A - QR section
and I don't know two questions (and can't understand the explination)

Care to help? please??

Q19) Kate has 6 different books. Assuming the order of the selection doesn't matter, how many ways can Kate select from 6 books, 2 different books.
- the answer said 6!/(4!2!) = 15
I am not sure where the (4!2!) came from....can you explain me??


21) Sales reduction forces a factory to cut back its output by 20%. by what percentage must the reduced sales be increased to allow production to be brought back to normal?
- Explanation from Barron's: Output reduction by 20% is akin to a decrease from 5 to 4. To return to a production of 5 from 4 is 25%

I am not sure what the heck it is saying and why it is the answer..I am not sure how to understand this..so can you explain me this in normal, easier way??

Thank you!


ps. Is Barron's harder than other test materials?? I getting 21-ish on Barrons on most of it and I am not sure I picked an easy one or not (Barron is not popular source...but it was cheapest in the bookstore..)

 

[dshope]

for qustion 19
you would need to know the difference between combination and permutation.
combination is used when the order does not matter
and permutation is used when the order matters

In this case, the problem says that the order does not matter and we are choosing 2 out of 6 books
so the answer would be 6C2 which is 6!/ (4!2!)

it might help if you google combination and permutation and study it!.

for 21)

lets say the original price was X.
with 20% price reduction the price now will be 0.8X
If we want the price to go back to X from 0.8X
we need to mulitply (X/0.8X) 1.25
which means 25% increase

hope this helped!

 

[Waheguru]

For Q19:
I did not look at the difference between permutation and combination but simply
Chalked out the situation and noted a pattern. Imagine 6 books, with each assigned a number from 1 to 6. I then realized we can have the following
pairs:
1, 2
1, 3
1, 4
1, 5
1, 6
2, 3
2, 4
2, 5
2, 6
3, 4
3, 5
3, 6
4, 5
4, 6
This is a total of 15 pairs.