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question:A bag contains 3 red marbles, 6 white marbles, and 8 blue marbles. Four marbles are drawn from the bag (without replacement). What is the probability that they are all the same color?
answer:First, we calculate the total number of ways to draw 4 marbles from the bag: [ text{Total ways} = binom{17}{4} = frac{17 times 16 times 15 times 14}{4 times 3 times 2 times 1} = 2380 ] Next, calculate the probability of drawing all red, all white, or all blue: - All red: ( binom{3}{4} = 0 ) (impossible to draw 4 reds as there are only 3) - All white: ( binom{6}{4} = 15 ) - All blue: ( binom{8}{4} = 70 ) Now, sum the probabilities: [ P(text{all same color}) = frac{binom{6}{4} + binom{8}{4}}{binom{17}{4}} = frac{15 + 70}{2380} = frac{85}{2380} ] Simplifying the fraction: [ frac{85}{2380} = frac{17}{476} ] Conclusion: [ boxed{frac{17}{476}} ]
question:A random sample of 10 households was taken from a residential area, and the monthly income data {x_i} (in units of thousand yuan) of the ith household was obtained. The following statistical values were calculated: sum_{i=1}^{10} x_i = 80, sum_{i=1}^{10} y_i = 20, sum_{i=1}^{10} x_i y_i = 184, and sum_{i=1}^{10} x_i^2 = 720. (1) Find the linear regression equation hat{y} = hat{b}x + hat{a} of the monthly savings y with respect to the monthly income x, and determine whether the variables x and y are positively or negatively correlated. (2) Predict the monthly savings of a household in this residential area with a monthly income of 7 thousand yuan.
answer:(1) From the given data, we have n=10, bar{x} = frac{1}{n} sum_{i=1}^{10} x_i = frac{80}{10} = 8, bar{y} = frac{1}{n} sum_{i=1}^{10} y_i = frac{20}{10} = 2. Next, we calculate the slope of the linear regression equation: hat{b} = frac{sum_{i=1}^{10} x_i y_i - nbar{x}bar{y}}{sum_{i=1}^{10} x_i^2 - nbar{x}^2} = frac{184 - 10 times 8 times 2}{720 - 10 times 8^2} = frac{24}{80} = 0.3 Then, we calculate the intercept of the linear regression equation: hat{a} = bar{y} - hat{b} bar{x} = 2 - 0.3 times 8 = -0.4 Therefore, the linear regression equation is hat{y} = 0.3x - 0.4. Since the slope hat{b} = 0.3 > 0, the variables x and y are positively correlated. (2) To predict the monthly savings of a household with a monthly income of 7 thousand yuan, we substitute x=7 into the linear regression equation: hat{y} = 0.3 x - 0.4 = 0.3 times 7 - 0.4 = 1.7 Therefore, the predicted monthly savings of a household with a monthly income of 7 thousand yuan is boxed{1.7} thousand yuan.
question:Alicia now earns 25 per hour, and 2% of this is deducted to pay local taxes. How many cents per hour of Alicia's wages are used to pay local taxes? mathrm{(A) } 40 text{ cents} qquad mathrm{(B) } 45 text{ cents} qquad mathrm{(C) } 50 text{ cents} qquad mathrm{(D) } 55 text{ cents} qquad mathrm{(E) } 60 text{ cents}
answer:1. **Convert Alicia's new hourly wage to cents**: Since Alicia earns 25 per hour, converting this to cents: [ 25 text{ dollars} times 100 text{ cents/dollar} = 2500 text{ cents} ] 2. **Calculate the new tax deduction in cents**: With the local tax rate now at 2%, we calculate the amount deducted for taxes from Alicia's wage in cents as follows: [ 2% text{ of } 2500 text{ cents} = 0.02 times 2500 = 50 text{ cents} ] 3. **Conclusion**: Thus, 50 cents per hour of Alicia's wages are used to pay local taxes. [ 50 text{ cents} ] The final answer is boxed{mathrm{(C) } 50 text{ cents}}
question:On a table, there is a certain quantity ( N ) of candies. Aline and Bruna agree that, alternately, each must eat at least one but no more than half of the existing quantity. The winner of the game is the one who eats the last candy. Aline always starts the game. a) For ( N = 5 ), which of the two has the winning position? (A winning position is one in which the player can ensure a win regardless of the opponent's sequence of moves.) b) For ( N = 20 ), which of the two has the winning position? c) What are the values of ( N ), where ( 100 < N < 200 ), that give Bruna the winning position?
answer:Let's address each part of the problem step-by-step. Part a) For ( N = 5 ) We need to determine if Aline, who starts the game, or Bruna, who plays second, is in the winning position when there are initially 5 candies. The rules are that each player must eat between 1 and half of the candies on the table. 1. A player wins if they can force the number of candies left to be exactly 1 on their opponent's turn. 2. To force the opponent into a losing position with exactly 2 candies, you need to leave them exactly 2 candies. 3. From a starting position of 5 candies: - If Aline eats 1 candy, there will be 4 candies left. Bruna then can eat up to 2 candies, leaving between 2 and 3 candies. - If Bruna eats 1 candy, she leaves exactly 3 candies (she could eat at most 2 candies). In both scenarios where Aline eats either 1 or 2 candies, Bruna can adjust her moves to force a position where Aline's subsequently forced move will allow Bruna to win. **Conclusion:** Bruna is in the winning position. boxed{text{Bruna}} Part b) For ( N = 20 ) We need to consider how Aline could force Bruna into a losing position. 1. If there are 20 candies initially, Aline plays first. 2. From experience (or previous knowledge), we know losing positions follow numbers of the form ( 2k + 1 ) where ( k ) was a previous losing position. From part a), we established 5 as a losing position. The next losing positions would be compiled similarly: - ( k = 2 rightarrow 2 cdot 2 + 1 = 5 ) - ( k = 5 rightarrow 2 cdot 5 + 1 = 11 ) - ( k = 11 rightarrow 2 cdot 11 + 1 = 23 ) (thus surpassing 20) 3. For Aline to force Bruna into ( 11 ) candies, she must eat: - ( 9 ) candies making ( 20 - 9 = 11 ) With Bruna stuck at 11 candies, following through the sequence will then demonstrate more easily that Bruna’s forced subsequent plays will lose to Aline. **Conclusion:** Aline is in the winning position. boxed{text{Aline}} Part c) For ( 100 < N < 200 ), find ( N ) where Bruna wins We seek all values ( N ) between 100 and 200 ensuring Bruna is in the winning position: Bruna wins if the initial value is a losing position for Aline. 1. The known series follows: 2, 5, 11, 23, 47, 95, 191, ldots 2. Locate values in provided interval ( 100 < N < 200 ): - The value falling within the interval is ( 191 ) from the compiled sequence. **Conclusion:** The values where Bruna wins are constrained to singleton 191 within our bounds. boxed{191}