英语翻译1 in how many of integers between 1 and 100 does the digit 5 occur exactly once?A 10 B 18 C 19 D 20 E 21 为什么选B?2 What is the total number of different 5-digit numbers that contain all of the digis 2,3,4,7 and 9 and in which none o

英语翻译1 in how many of integers between 1 and 100 does the digit 5 occur exactly once?A 10 B 18 C 19 D 20 E 21 为什么选B?2 What is the total number of different 5-digit numbers that contain all of the digis 2,3,4,7 and 9 and in which none o
英语翻译
1 in how many of integers between 1 and 100 does the digit 5 occur exactly once?
A 10 B 18 C 19 D 20 E 21 为什么选B?
2 What is the total number of different 5-digit numbers that contain all of the digis 2,3,4,7 and 9 and in which none of the odd digits occur next to each other?
A 12 B 10 C 8 D 6 E 1 为什么选A?此题没读懂?
3 the value of y will decrease as the value of x increases in which of the following equations?
A y=x B y=x-2 C 2-y=3-x D 2x-y=4 E x+2y=1 为什么选E而不选A?
4 A rectangular playing field is 40 meters longer but 30 meters narrower than a nearby square playing field.How many more meters of fencing would be needed to enclose the rectangular field than to enclose the square field?
A 70 B 50 C 40 D 20 E 10 为什么选D?
5 A rectangular solid is twice as long as it is wide.The height of the solid is the average (arithmetic mean) of the length and the width,What is the ratio of the length to the height of the solid?
A 2 to 3 B 3 to 1 C 3 to 2 D 3 to 4 E 4 to 3 为什么选E?

英语翻译1 in how many of integers between 1 and 100 does the digit 5 occur exactly once?A 10 B 18 C 19 D 20 E 21 为什么选B?2 What is the total number of different 5-digit numbers that contain all of the digis 2,3,4,7 and 9 and in which none o
1.
5,15,25,直到95,一共10个,减去55（因为5只能出现一次,所以55不要）是9个.
50-59,共10个,减去55是9个.
9+9=18个
2.一个五位数由2、3、4、7和9组成,而奇数不能相连,能有几个这种五位数?
3、7、9是奇数,不能相连所以必须在第1、3、5（个、百、万）位上.
组合的可能数为3x2x1 = 6.
2和4在十位和千位上可互换,于是6x2 = 12.
一共12种可能.
3.问的是x增大时y减小啊,A肯定不行.
只有E是对的,y=(1-x)/2
4.栏杆等于周长.
设square边长x米.
square的栏杆用4x米,
rectangle的栏杆用2 *（x+40+x-30) = 4x + 20
所以rectangle多用20.
5.
设长方形短边为x,则长边为2x.
高为 (x + 2x) / 2 = 1.5x.
长边与高的比为 2x/ 1.5x = 4:3