Kaplan SAT Subject Math Level 1-Practice 4

 

Better solution is xy = 28, xz = 20, yz = 35, then (xyz)^2 = 28 * 20 * 35, square root is 140. It is the answer. 

Q9: Within a given area code, how many seven-digit numbers are available, given the restriction that the first three digits cannot be 911 or 411 and the first digit cannot be a 1 or a 0?

My solution is : total options is 10 * 10 * 10 * 10 * 10 * 10 * 10 = 10^7

since first digit cannot be a 1 or a 0, then minus  10 * 10 * 10 * 10 * 10 * 10  (the first digit must be 1) and 10 * 10 * 10 * 10 * 10 * 10 ( (the first digit must be 0). It will be 8 * 10 * 10 * 10 * 10 * 10 * 10. 

When the first three digits must be 911, it has 10 * 10 * 10 * 10. 

When the first three digits must be 411, it has 10 * 10 * 10 * 10. 

Thus, final result is  8 * 10 * 10 * 10 * 10 * 10 * 10 - 10 * 10 * 10 * 10 - 10 * 10 * 10 * 10 = 7,980,000

I doubt my solution since I don't think it could involve so many steps. But it turns out it has to be.

What I did is to try all choices such that a * 1/a = 1.

先算三个交集的人数，再用总数减去这个数既是答案。

（18－10）+ （ 25－10） + （15－10） +10 = 38

50－38 = 12

The median is not average of 9th and 10th numbers