## Thursday, December 29, 2016

### 2013 Mathcounts School and Chapter Harder Problems

Here is the link to the official Mathcounts website.

Some more challenging problems from this year's Mathcounts school/or chapter problems.

2013 school team #10 : Three concepts are testing here :
Hint:
a. If you get rid of the remainder, the numbers will be evenly divided into 192, so you are looking at
those factors of 192 - 12 = 180

b. To leave a remainder of 12, those factors of 180 that are included in the Set must be smaller than 12, otherwise, you can further divide it.

c. To find the median, make sure to line up the numbers from the smallest to the largest and find the middle numbers. If there are even numbers of factors larger than 12, average the middle two. Otherwise, the middle number is the answer.

$$180=2^{2}\times 3^{2}\times 5$$ so there are (2 + 1) (2 + 1) (1 + 1) = 18 factors

The list on the left side gives you the first 9 and if you times those numbers with "5", you get 9 other factors,which are 5, 10, 20, 15, 18, 60, 45, 90 and 180.

Discard the factors that are smaller or equal to 12 and list all the other factors in order and find the median.

2013 Chapter Sprint:
#21: Dimensional change problem : The height of the top pyramid is $$\dfrac {2} {3}$$ of the larger
pyramid so its volume is $$\left( \dfrac {2} {3}\right) ^{3}$$ of the larger pyramid.

$$\left( \dfrac {2} {3}\right) ^{3}\times \dfrac {1} {3}\times \left( \dfrac {36} {4}\right) ^{2}\times 12 =$$ $$96 cm^{3}$$

# 24:  According to the given:   $$xyz=720$$   and   $$2( xy+yz+zx)= 484$$ so
$$( xy+yz+zx )= 242$$

Since x, y and z are all integers, you factor 720 and see if it will come up with the same x, y and z values
for the second condition.

Problem writer(s) are very smart using this number because the numbers "6", "10", "12" would give you
a surface area of 252. (not right)

The three corrrect numbers are "8", "9", and "10" so the answer is $$\sqrt {8^{2}+9^{2}+10^{2}}=$$ $$7\sqrt {5}$$

#25: Geometric probability: Explanations to similar questions and more practices below.

Probability with geometry representations form Aops.

Geometric probability from "Cut the Knots".

#26: This one is similar to 2002 AMC-10B #21, so try that question to get more practices.

#27:
$$\dfrac {1} {A}+\dfrac {1} {B}=\dfrac {1} {2}$$
$$\dfrac {1} {B}+\dfrac {1} {C}=\dfrac {1} {3}$$
$$\dfrac {1} {C}+\dfrac {1} {A}=\dfrac {1} {4}$$
Add them up and you have  $$2 * (\dfrac {1} {A}+\dfrac {1} {B}+ \dfrac {1} {C})=\dfrac {13} {12}$$

$$(\dfrac {1} {A}+\dfrac {1} {B}+ \dfrac {1} {C})=\dfrac {13} {24}$$

$$\dfrac {1} {\dfrac {1} {A}+\dfrac {1} {B}+\dfrac {1} {C}} =$$$$\dfrac {{24}} {13}$$ hours

#28: Hint : the nth triangular number is the sum of the first "n" natural numbers and $$\dfrac {n\left( n+1\right) } {2}$$ is how you use to find the sum.
From there, you should be able to find how many numbers will be evenly divided by "7".

#29 : Circle questions are very tricky so make sure to find more problems to practice accuracy.

#30 :
Solution I:
Read the solution that is provided by Mathcounts.org here.
Solution II:
Case 1 : $$x-1 > 0\rightarrow x > 1$$ Times ( x - 1) on both sides and you have
$$x^{2}-1>8$$ so x > 3 or x < -3 (discard)

Case 2: $$x-1 < 0$$ so $$x < 1$$ $$\rightarrow x^{2}-1 < 8$$ [You need to change the sign since it's negative.]-3 < x < 3. Combined with x < 1 you have the range as -3 < x < 1

2013 Mathcounts Target #7 and 8:

Target question #8 is very similar to 2011 chapter team #10