10 different biology books and 8 different chemistry books lie on a shelf. In how many ways can a student pick 2 books of each type?
A. 80
B. 160
C. 720
D. 1100
E. 1260
OA E
Source: Economist Gmat
10 different biology books and 8 different chemistry books l
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- fskilnik@GMATH
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$$? = C\left( {10,2} \right) \cdot C\left( {8,2} \right) = \left( {\frac{{10 \cdot 9}}{2}} \right) \cdot \left( {\frac{{8 \cdot 7}}{2}} \right) = 45 \cdot 28 = \underleftrightarrow {45 \cdot \left( {30 - 2} \right) = 1350 - 90} = 1260$$BTGmoderatorDC wrote:10 different biology books and 8 different chemistry books lie on a shelf. In how many ways can a student pick 2 books of each type?
A. 80
B. 160
C. 720
D. 1100
E. 1260
Source: Economist Gmat
We follow the notations and rationale taught in the GMATH method.
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Take the task of selecting 4 books and break it into stages.BTGmoderatorDC wrote:10 different biology books and 8 different chemistry books lie on a shelf. In how many ways can a student pick 2 books of each type?
A. 80
B. 160
C. 720
D. 1100
E. 1260
Stage 1: Select two biology books
Since the order in which we select the books does not matter, we can use combinations.
We can select 2 books from 10 books in 10C2 ways (45 ways)
So, we can complete stage 1 in 45 ways
If anyone is interested, we have a free video on calculating combinations (like 10C2) in your head: https://www.gmatprepnow.com/module/gmat ... /video/775
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Brent
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Since the order in which the books are selected does not matter, we have a combination problem. Let's start by determining the number of ways to select the biology books.BTGmoderatorDC wrote:10 different biology books and 8 different chemistry books lie on a shelf. In how many ways can a student pick 2 books of each type?
A. 80
B. 160
C. 720
D. 1100
E. 1260
OA E
Source: Economist Gmat
Number of ways to select the biology books = 10C2 = 10!/(2! x 8!) = (10 x 9)/2! = 5 x 9 = 45.
Next we can determine the number of ways to select the chemistry books.
Number of ways to select the chemistry books = 8C2 = 8!/(2! x 6!) = (8 x 7)/2! = 4 x 7 = 28.
Thus, the number of ways to select 2 biology books and 2 chemistry books is 45 x 28 = 1,260.
Answer: E
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