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25: Find the value of x if:

　　x + y + z = 5

　　x + y - z = 3

　　x - y = 2

　　(a) -3

　　(b) -1

　　(c) 0

　　(d) 3

　　(e) 5

Answer:

We notice that if we add equations 1 and 2 we can eliminate z: x + y + z + x + y - z = 5 + 3

2·(x + y) = 8

x + y = 4

At this point we have a system of 2 linear equations with 2 variables, x and y:

x - y = 2

x + y = 4

If we add these 2 equations, we get 2·x = 6 so x = 3.

26:What is 2% of 8%?

　　(a) .0016%

　　(b) .016%

　　(c) .16%

　　(d) 1.6%

　　(e) 16%

Answer:

8% of the value x is (8100)·x

2% of (8100)·x is (2100)·(8100)·x = (.016100)·x

The answer is .16%.

延伸阅读：

SAT数学专项练习题（九）

SAT数学专项练习题（十一）

SAT数学专项练习题（十二）

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