You have accidentally left out the plug and are attempting to 
fill the bath with both 2 taps full on.  The hot tap takes 6 
minutes to fill the bath, the cold tap takes 4 minutes, and the 
water empties through the plug hole in 12 minutes.  In how many 
minutes will the bath be filled?  

3 minutes. This is solved by
1/6 + 1/4 - 1/12 = 0.166 + 0.25 - 0.083 = 0.333
1/0.333 = 3 minutes 

 A man is wearing black. Black shoes, socks, trousers, jumper, 
gloves, and balaclava.  He is walking down a black street
with all the street lamps off.  A black car is coming toward him 
with its lights off too, but somehow manages to stop in time.  How 
did the driver see the man?

It was day time.

28 days. Once it reaches the top, it does not slip down 2 feet. 

 Two men were being tried for murder. The jury found one man guilty 
and the other innocent. The judge turned to the guilty man 
and said, "Even though your guilt has been established, the law 
compels me to set you free." How could such a bizarre judgement occur? 

They were siamese twins!

You have two hourglasses--a 4-minute glass and a 7-minute glass. 
You want to measure 9 minutes. How do you do it?

Start both hourglasses. When the 4-minute glass runs out, turn it 
over (4 minutes elapsed). When the 7-minute glass runs out, turn 
it over (7 minutes elapsed). When the 4-minute glass runs out this 
time (8 minutes elapsed), the 7-minute glass has been running for 
1 minute. Turn it over once again. When it stops, 9 minutes have 
elapsed. 

They were two of a set of triplets (or quadruplets, etc.

 A boat is at anchor. Over the side hangs a rope ladder 
with rungs a foot apart. The tide rises at a rate of 10 inches per hour. 
At the end of six hours, how much of the rope ladder will remain 
above water, assuming that 10 feet were above the water when the tide 
began to rise?

Since the boat is afloat, the water level in 
relation to the ship stays the same. Therefore, 10 feet are above water, 
just as the beginning!