又是数学问题!求救!

shayuamerica

1:

In Town X, 64 percent of the population are employed, and 48 percent of the population are employed males.  What percent of the employed people in Town X are females?

答案为什么是25%

2:

A total of $60,000 was invested for one year.  But of this amount earned simple annual interest at the rate of x percent per year, and the rest earned simple annual interest at the rate of y percent per year.  If the total interest earned by the $60,000 for that year was $4,080, what is the value of x?

(1)     x = (3/4) y

(2)   The ratio of the amount that earned interest at the rate of x percent per year to the amount that earned interest at the rate of y percent per year was 3 to 2.

答案是C但是我没有头绪,希望牛牛不吝赐教!

3:

In the sequence 1, 2, 4, 8, 16, 32, …, each term after the first is twice the previous term.  What is the sum of the 16th, 17th, and 18th terms in the sequence?

KEY:

A.      7(2¹⁵)

WHY?

A.      7(2¹⁵)

WHY?

A total of $60,000 was invested for one year.  But of this amount earned simple annual interest at the rate of x percent per year, and the rest earned simple annual interest at the rate of y percent per year.  If the total interest earned by the $60,000 for that year was $4,080, what is the value of x?

(1)     x = (3/4) y

(2)   The ratio of the amount that earned interest at the rate of x percent per year to the amount that earned interest at the rate of y percent per year was 3 to 2.

答案是C但是我没有头绪,希望牛牛不吝赐教!

3:

In the sequence 1, 2, 4, 8, 16, 32, …, each term after the first is twice the previous term.  What is the sum of the 16th, 17th, and 18th terms in the sequence?

KEY:

A.      7(2¹⁵)

WHY?

A.      7(2¹⁵)

WHY?

liujingsheng

第一题，（0.64-0.48）/0.64=0.4

第二题，三个未知数，三个方程联力求解！

第三题，a1=1,q=2 

a16=2^15

a17=2^16

a18=2^17

sum=2^15*7

shayuamerica

对哦!看来我都做傻了!呵呵!