stucashS

4. an integer greater than 1 that is not prime is called composite. if the two-digit integer n is greater than 20, is n composite?
1) the tens digit of n is a factor of the units digit of n.
2) the tens digit of n is 2.

1) Let's say the units digit of n is a.  Then the tens digit of n is a factor of a.  Therefore, n = 10*m*a + a = a*(10m +1).  Then n has a factor of a and a is bigger than 1 since n is greater than 20.  n is a composite for sure. Sufficient.
2) Insufficient.  21, 22, 23 ....

stucashS

5. is z equal to the median of the three positive integers x,y, and z?
1) x<y+z
2) y=z

1) Insufficient.  x=1, y=2, z=3.  x=1, y=3, z=2.
2) Sufficient. Either x, y=z; or y=z; x.

stucashS

If you can see the attached picture, you would have a better time to solve the problem 1 LZ posted.

According to the stimulus, [9] = 3*9 =27. [6] = (1/2) * 6 = 3.  So [9]*[6] = 27 * 3 = 81.  So far so good.  But if you choose (a) as the correct answer, you were wrong because (a) is [81] = 3*81 = 243!

On the other hand, (d) [27] = 3*27 = 81, is the right answer!