Solution:

2, 3, 10 and 10
To maintain the difference of eight and keeping in mind the third and fourth hint, the first and third cards can be 2 and 10 or 10 and 2. But keeping in mind the first hint, we know that the first card is 2 and the third is 10. Similarly, if we see the fifth hint, the difference between the second and fourth card should be 7. Thus we know that the second card is 3 and the fourth card is 10.

Solution:

I filled a glass full of water and held it over my head for 20 minutes.

Solution:

13 x 3 + 1 = 40

The number of decks is absolutely not relevant here which means whether we mix 5 or 500 cards still results would be same.
Any card drawn will be a Ace,2,3,4,5,6,7,8,9,10,Jack,Queen or King, so there are 13 possibilities each time a card is drawn.
If you are lucky just 5 cards of the same kind can be obtained in 4 steps

The unluckiest(worst) way is our solution as we need to guarantee a four of a kind.
Draw 3 of each kind =>now we have 39 cards. Next card will guarantee 4 of a kind.

Solution:

28 =>1+2+4+7+14

cannot be 1 (as number cannot be included)

Solution:

From top to bottom, the flavors are :

nutty-chocolate
peppermint
orange
Jamaica
walnut

Solution:

Old password : altered

New Password : distinct

Solution:

There are 3 fruits (1 grapes, 1 apples, and 1 oranges).

Solution:

2 people

Say 2 helpers are Helper1 & Helper2

Helper1 food Helper2 food McDuck's Food
End of Day 1 : 1 4 4
Now 'A' will head back

End of Day 2 : N/A 2 4
Now 'B' will head back

Now Mr. McDuck have 4 day food :-)

Solution:

Move and then pour all juice from second glass to fifth glass.

Solution:

C

Explanation:
It Seems, the water level might be even in all the glasses, but what will happened if we remove the objects from the glasses. When you put an object in water, it will displace the water with the mass of the object Hence, the glass that contains the object with the least amount of weight will have the most water in it.

Even though we might not know the exact weight of all those four objects, we can all agree, that the paper clip weighs the least and hence will displace the minimum water.