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

Solution:

The murderer is Marcus.

The notes hinted towards the name of Marcus. How?
Just take the first letter from Compound, Art Room, Restroom, Underwater Room, Study Room and Movie Room and you will get:

C, A, R, U, S and M.

Rearranging these words leads to the name Marcus who is the murderer.

Solution:

She was not able to find 11 on phone.

Solution:

The next number is 392.

The first number = (Square of 1 + Cube of 1)
The second number = (Square of 2 + Cube of 2)
The third number = (Square of 3 + Cube of 3)
The fourth number = (Square of 4 + Cube of 4)
The fifth number = (Square of 5 + Cube of 5)
The sixth number = (Square of 6 + Cube of 6)
Therefore, the seventh number will be
Square of 7 + Cube of 7 = 392.

Solution:

The password that opens the chamber is 65292.