Problem Solving

1)
If 10 persons meet at a reunion and each person shakes hands exactly once with each of the others, what is the total number of handshakes?
(A) 10•9•8•7•6•5•4•3•2•1
(B) 10•10
(C) 10•9
(D) 45
(E) 36

OA: D
I thought it will be 10•9, since each person shakes hand 10 times with everyone. So 10*9.

2)
Trucking transportation rates are x dollars per metric ton per kilometer. How much does it cost, in dollars, to transport one dozen cars, which weigh two metric tons each, n kilometers by truck?

(A) x/12n
(B) x/24n
(C) xn/24
(D) 12xn
(E) 24xn

OA: E
This one I just don't get it. I thought the answer would be x/24n

3)
One-fifth of the light switches produced by a cer¬tain factory are defective. Four-fifths of the defective switches are rejected and of the nondefective switches are rejected by mistake. If all the switches not rejected are sold, what percent of the switches sold by the factory are defective?

(A) 4%
(B) 5%
(C) 6.25%
(D) 11%
(E) 16%

OA: B
Again no clue on this one

4)
What is the least possible product of 4 different integers, each of which has a value between –5 and 10, inclusive?
(A) –5040 (B) –3600
(C) –720 (D) –600
(E) –120

OA: B
No idea.

5)
If a motorist had driven 1 hour longer on a certain day and at an average rate of 5 miles per hour faster, he would have covered 70 more miles than he actually did. How many more miles would he have covered than he actually did if he had driven 2 hours longer and at an average rate of 10 miles per hour faster on that day?
(A) 100 (B) 120 (C) 140
(D) 150 (E) 160

OA: D
No idea here too.

Thank you in advance for your help.