#### 题目列表

What is the ratio of the number of months for which the percent increase from 2015 to 2016 in the average daily electric use was greater than 28 percent to the number of months for which the percent decrease from 2015 to 2016 in the average daily electric use was greater than 10 percent?

The standard deviation of n numbers $x_{1}$, $x_{2}$, $x_{3}$,......, $x_{n}$, with mean x is equal to $\sqrt{\frac{s}{n}}$, where S is the sum of the squared differences, $(x_{i}, - x)^{2}$ for 1 ≤ i ≤ n.

If the standard deviation of the 4 numbers 140-a, 140, 160, and 160+a is 50, where a > 0, what is the value of a?
In sequence T, each term after the first term is d more than the preceding term. The sum of the first 10 terms of T is 210. The sum of the first 20 terms of T is 820. What is the value of d?
In two equilateral parallelograms, X and Y, the sum of the lengths of the diagonals of X is equal to the sum of the lengths of the diagonals of Y. In X, the length of the longer diagonal is 20 more than the length of the shorter diagonal. In Y, the length of the longer diagonal is 8 more than the length of the shorter diagonal. What is the area of Y minus the area of X?
1, 2, 2, 3, 3, 3, 4, 4, 4, 4, ......, 24

The list above consists of 300 entries that are integers in increasing order, and each integer n occurs n times for 1≤n≤24. What is the least integer k in the list for which at least 15% of all the entries are less than k?
The probability distribution function P of a continuous random variable X is defined as shown. If P(X≤k) < $\frac{1}{2}$ , which of the following could be the value of k?

Indicate all such values. #### Quantity A

$89!-88!$

#### Quantity B

$(87!)(88)^{2}$

How many integers between 100 and 1,000 are multiples of 7?
The standard deviation of m numerical data $x_{1}, x_{2}, x_{3}, ..., x_{n}$, With mean $\bar{x}$ is equal to $\sqrt{\frac{S}{n}}$, where S is the sum of the squared differences $(x_{i}-\tilde{x})^{2}$, for 1 ≤ i ≤ n.

On a certain examination, 7 students received scores of 85, 90, 70, 90, 75, 90, and 95. For the 7 scores, the mode was approximately how many standard deviations above the mean?
n, k and r are all positive integers

If $n^{k}$=10r+3, then n could be?

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