Appearance
question:the final round of a giraffe beauty contest, two giraffes named Tall and Spotted have made it to this stage. There are 105 voters divided into 5 districts, each district divided into 7 sections, with each section having 3 voters. Voters select the winner in their section by majority vote; in a district, the giraffe winning the majority of sections wins the district; finally, the giraffe winning the majority of districts is declared the contest winner. The giraffe named Tall won. What is the minimum number of voters who could have voted for him?
answer:1. **Understanding the structure of voting:** - Total number of voters: 105. - The 105 voters are divided into 5 districts: [ text{Number of voters per district} = frac{105}{5} = 21 ] - Each district is divided into 7 sections: [ text{Number of voters per section} = frac{21}{7} = 3 ] 2. **Determining the requirements for winning:** - To win the final, the giraffe "Vyisokiy" must win in the majority of the districts, i.e., at least 3 out of 5 districts. - To win a district, "Vyisokiy" must win in the majority of the sections in that district, i.e., at least 4 out of 7 sections. 3. **Calculating the number of sections needed:** - To secure a district: [ 4 text{ sections} ] - To secure 3 districts: [ 3 cdot 4 = 12 text{ sections} ] 4. **Number of voters required per section:** - To win in a section, at least 2 out of 3 voters must vote for "Vyisokiy." 5. **Calculating the minimal number of voters needed:** - For 12 sections: [ 12 cdot 2 = 24 text{ voters} ] 6. **Verification by valid example generation:** - The calculation ensures that with 24 voters, strategically placed, "Vyisokiy" can win the required 12 sections across 3 districts to become the final winner. Concluding from the above reasoning, the minimum number of voters needed to ensure "Vyisokiy's" victory is: [ boxed{24} ]
question:The average weight of A, B, and C is 84 kg. If D joins the group, the average weight of the group becomes 80 kg. If another man E, who weighs some kg more than D, replaces A, then the average of B, C, D, and E becomes 79 kg. How much more does E weigh than D if the weight of A is 77 kg?
answer:Let's denote the weights of A, B, and C as ( W_A, W_B, ) and ( W_C ) respectively. According to the information given: 1. The average weight of A, B, and C is 84 kg: [ frac{W_A + W_B + W_C}{3} = 84 ] [ W_A + W_B + W_C = 84 times 3 ] [ W_A + W_B + W_C = 252 ] 2. When D joins the group, the average weight becomes 80 kg: [ frac{W_A + W_B + W_C + W_D}{4} = 80 ] [ W_A + W_B + W_C + W_D = 80 times 4 ] [ W_A + W_B + W_C + W_D = 320 ] From the first equation, we know that ( W_A + W_B + W_C = 252 ), so we can find the weight of D by subtracting 252 from 320: [ W_D = 320 - 252 ] [ W_D = 68 ] 3. If E replaces A, the average weight of B, C, D, and E becomes 79 kg: [ frac{W_B + W_C + W_D + W_E}{4} = 79 ] [ W_B + W_C + W_D + W_E = 79 times 4 ] [ W_B + W_C + W_D + W_E = 316 ] We know that ( W_B + W_C = 252 - W_A ), and since the weight of A is 77 kg: [ W_B + W_C = 252 - 77 ] [ W_B + W_C = 175 ] Now we can find the weight of E by substituting ( W_B + W_C ) and ( W_D ) into the equation: [ 175 + 68 + W_E = 316 ] [ 243 + W_E = 316 ] [ W_E = 316 - 243 ] [ W_E = 73 ] Now we can find out how much more E weighs than D: [ W_E - W_D = 73 - 68 ] [ W_E - W_D = 5 ] So, E weighs boxed{5} kg more than D.
question:For all complex numbers ( z ), let [ f(z) = left{ begin{array}{cl} z^{2} & text{if } z text{ is nonreal}, z^2 & text{if } z text{ is positive real}, z^3 & text{if } z text{ is negative real}. end{array} right. ] Find ( f(f(f(f(1+i)))) ).
answer:Step by step, we handle the function: 1. Since ( 1+i ) is non-real, ( f(1+i) = (1+i)^2 = 1 + 2i - 1 = 2i ). 2. ( 2i ) is non-real, so ( f(2i) = (2i)^2 = -4 ). 3. ( -4 ) is negative real, applying the new function rule, ( f(-4) = (-4)^3 = -64 ). 4. ( -64 ) is negative real, so ( f(-64) = (-64)^3 = -262144 ). In each case, the correct rule was applied based on the new definition of ( f ). Therefore: (boxed{-262144})
question:Let ( S ) be a set consisting of ( m ) pairs ((a, b)) of positive integers with the property that ( 1 leq a < b leq n ). Show that there are at least [ 4 m frac{left(mfrac{n^{2}}{4}right)}{3 n} ] triples ((a, b, c)) such that ((a, b)), ((a, c)), and ((b, c)) belong to ( S ).
answer:1. **Definition and Setup**: Let a triple (a, b, c) be termed **good** if and only if (a, b), (a, c), and (b, c) are all pairs in the set S. Define d_i as the number of pairs in S containing the integer i and let D_i be the set of numbers paired with i in S (so left|D_i right| = d_i). 2. **Counting Triple Intersections**: For any pair (i, j) in S, we want to estimate the number of integers k such that {i, j, k} is good. This is equivalent to finding [ left| D_i cap D_j right| ] Note that i notin D_i and j notin D_j. Consequently, neither i nor j can be in D_i cap D_j, ensuring that any k in D_i cap D_j is different from both i and j. 3. **Bounding the Intersection**: Since D_i cup D_j subseteq {1, 2, ldots, n}, we use this to find: [ left| D_i cap D_j right| = left| D_i right| + left| D_j right| - left| D_i cup D_j right| ] We have: [ left| D_i cup D_j right| leq n ] Thus, [ left| D_i cap D_j right| leq d_i + d_j - n ] 4. **Summing Over All Pairs**: Consider all pairs (i, j) in S. Each good triple (a, b, c) will be counted three times (once for each pair of the three elements in the triple). Let T be the total count of good triples. We sum up over the pairs: [ T geq frac{1}{3} sum_{(i, j) in S} left( d_i + d_j - n right) ] 5. **Using Cauchy-Schwartz Inequality**: Rewriting the sum, we see that each d_i term appears d_i times (since it counts how many pairs include i). We write: [ T geq frac{1}{3} left( sum_{i=1}^{n} d_i^2 - m cdot n right) ] 6. **Applying Cauchy-Schwartz**: Using the Cauchy-Schwarz inequality: [ sum_{i=1}^{n} d_i^2 geq frac{left( sum_{i=1}^{n} d_i right)^2}{n} ] Thus: [ T geq frac{1}{3} left( frac{left( sum_{i=1}^{n} d_i right)^2}{n} - m cdot n right) ] 7. **Sum of d_i**: Observe that: [ sum_{i=1}^{n} d_i = 2m ] Each pair (a, b) contributes to both d_a and d_b, hence the total sum of d_i counts twice the number of pairs. 8. **Final Calculation**: Substitute this back in: [ T geq frac{1}{3} left( frac{(2m)^2}{n} - mn right) = frac{1}{3} left( frac{4m^2}{n} - mn right) ] Simplifying: [ T geq 4m frac{ m - frac{n^2}{4} }{3n} ] 9. **Conclusion**: [ boxed{ T geq 4m frac{ m - frac{n^2}{4} }{3n} } ] This concludes the detailed step-by-step solution, demonstrating that the number of good triples (a, b, c) is at least (boxed{4m frac{ m - frac{n^2}{4} }{3n}}).