prep math

weiqite

求教数学题目~~(prep)

1. If each item in the sum a1+a2....+an is either 7 or 77 and the sum equals to 350, which of the following could be equal to n?

a. 38, b 39, c, 40, d, 41, e, 42

怎么考虑这个题目哦？

2. If X and Y are positive integers, what is the value of xy?

(1) The greatest common factor of x and y is 10;

(2) The least common multiple X and y is 180.

答案是：both statements together are sufficient, but neither statement alone is sufficient.

最大公约数和最小公倍数嘛？有这个规定吗？数学忘记了。。。

3. From a bag containing 12 identical blue balls, y identical yellow balls, and no other balls, one ball will be removed at random, if the probability is less than 2/5 that the removed ball will be blue, what is the least number of yellow balls that must be in the bag?

a. 17, b 18,c, 19, d, 20, e, 21

谢谢~~

yang_jing_8

求教数学题目~~(prep)

1. If each item in the sum a1+a2....+an is either 7 or 77 and the sum equals to 350, which of the following could be equal to n?

a. 38, b 39, c, 40, d, 41, e, 42

怎么考虑这个题目哦？

Assume that there are x 7 and y 77 in the list. Then n=x+y

7x+77y=350--> x+11y=50 when y=1, x=39, then n=1+39=40; when y=2,x=28, then n=30; wheny=3,x=17, n=20 ;when y=4, x=6, n=10

yang_jing_8

2. If X and Y are positive integers, what is the value of xy?

(1) The greatest common factor of x and y is 10;

(2) The least common multiple X and y is 180.

Conbine condition 1 and 2
the numbers which is both a factor of 180 and a multiple of 10 are as follows: 10, 20, 30, 60,90,180
In this list, only 10 and 180 qualify both 1 and 2, then xy=1800

3. From a bag containing 12 identical blue balls, y identical yellow
balls, and no other balls, one ball will be removed at random, if the
probability is less than 2/5 that the removed ball will be blue, what
is the least number of yellow balls that must be in the bag?

a. 17, b 18,c, 19, d, 20, e, 21

The probability of draw a bule ball is  P=12/12+y<2/5; then y>18. So I think the answer should be C.

weiqite

谢谢指教~~~bless