Solution:

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

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:

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

Solution:

20741616380

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

Solution:

SC

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

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:

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:

When he is rowing the boat in the river, the boat is floating with watermelons in it and, therefore, all the watermelons have already displaced the water equal to their volume. Hence, there won't be any change in the water level, whether the farmer eats it or throws them away.

Solution:

Maths Teacher

Explanation:
How can she give grades on the 1st day of the college?

Solution:

As shown below, we can make equation as 6 = 6 = 6.