Problem Solving

Hey guys,
Came across the following questions in GMAT PREP. PLz help...

PS =>
1. Which of the following inequalities has a solution set that, when graphed on number line, is a single line segment of finite length?
1. x^4 >= 1
2. x^3 <= 27
3. x^2 >= 16
4. 2 <= mod “x” <= 5
5. 2 <= 3x + 4 <= 6
Answer = 5

2. If n is a multiple of 5 and n=(p^2)q, p and q are prime numbers, which of the following must be a multiple of 25?
1. p^2 * q^2
2. p^3 * q
3. pq
4. p^2
5. q^2
Answer = 1 (p^2 * q^2)

3. If x > y^2 > z^4
z > y > x could be true?
Answer = Yes (How???)

4. There are 4 senior managers & 6 junior managers. How many diff 3 member groups can formed in which at least one member in group is a senior manager?
1. 48
2. 100
3. 120
4. 288
5. 600
Answer = 100

5. Alice’s takehome pay last yr was same each month, and she saved same fraction each month from her takehome pay. The total amount she saved at end of tht yr was 3 times amount of the portion of her monthly takehome pay that she did not save. If all the money she saved last yr was from her takehome pay, wat fraction of her takehome pay did she save each month?
½, 1/3, ¼, 1/5, 1/6

Answer = 1/5 ( i get 1/4 )

Also, any recommendations for improvement in problems involving Number Properties & permutations & combinations ?

Thanks in advance