Solution:

Both will rotate at Same speed.

Explanation:
Gears of the same size will always rotate at constant speeds despite any smaller or larger gears in between them.

Solution:

Let's say the Mice are X and Y and Bottle are B1, B2....B9

Step1:
X drinks B1 B2 & B3
Y drinks B1 B4 & B5

Possible Outcomes
If both mice die, B1 is the poison bottle.
If only mice X dies, Either B2 or B3 is the poison bottle.
If only mice Y dies, Either B4 or B5 is the poison bottle.
If nobody dies, Poison is in B6 B7 B8 or B9


Step2: (If nobody dies)
X drinks B6 B7
Y drinks B6 B8

Possible Outcomes
If both mice die, B6 is the poison bottle.
If only mice X dies, B7 is the poison bottle.
If only mice Y dies, B8 is the poison bottle.
If nobody dies, B9 is the poison bottle.

Step2: (Only X dies - Either B2 or B3 is the poison bottle)
Y drinks B2, if Y dies B2 is the poison bottle else B3

Step2: (Only Y dies - Either B4 or B5 is the poison bottle)
X drinks B4, if X dies B4 is the poison bottle else B5

Solution:

We just added four matchsticks as shown in below picture.

Solution:

Let us assume,
the radius of the river is R.
the speed of beast is S.
the speed of duck is S/4.
Circumference = 2 * Pi * R.

Now, if duck swims R/4 distance from the center of the river and then begins to swim across the river in center implies both duck and beast can take the round trip in same time.
Explanation :
time by beast : 2 * Pi* R * S
time by duck : 2 * Pi* R/4 * S/4 => 2 * Pi* R * S


Now, the duck can move slowly inward toward the center of the river, and begin swimming around the center in a circle from this distance. It is now going around a very slightly smaller circle than it was a moment ago, and thus will be able to swim around this circle FASTER than the beast can run around the shore.

The duck can keep swimming around this way, pulling further away each second, until finally it is on the opposite side of its inner circle from where the beast is on the shore. At this point, the duck aims directly toward the closest shore and begins swimming that way. At this point, the duck has to swim [0.75R feet + 1 millimeter] to get to shore. Meanwhile, the beast will have to run R*pi feet (half the circumference of the river) to get to where the duck is headed.
The beast runs four times as fast as the duck, but you can see that it has more than four times as far to run:

[0.75R feet + 1 millimeter] * 4 < R*pi

[This math could actually be incorrect if R were very small, but, in that case, we could just say the duck swam inward even less than a millimeter and make the math work out correctly.]

Because the duck has less than a fourth of the distance to travel as the beast, it will reach the shore before the beast reaches where it is and successfully escape.

Solution:

Uncle Scrooge tells him he took the loan to keep his expensive car safe and he also saves money on security and parking rent.

Solution:

SC

12 zodiac signs are : Aries Taurus Gemini Cancer Leo Virgo Libra Scorpio Sagittarius Capricorn Aquarius Pisces

Solution:

20741616380

Explanation:
5 * 5 - 5 =>20
20 * 20 - 20 => 380
380 * 380 -380 =>144020
144020 * 144020 - 144020 => 20741616380

Solution:

"World War I" was named after after "World War II"

Solution:

common sense

Explanation :
We can break it into the following three parts
ITIT
,N
CENTCENT

Now ITIT = 2 Its which can be molded into IT's
,N = Comma + N which can be molded into Common
CENTCENT = 2 Cents which can be molded into Sense

So, the letters basically represent:
It's common sense.

Solution:

Were you able to do it under two minutes? The words that will be formed are as below:
Friend
Society
Alien
Stymie
Shriek