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:

Six.

There are only five colors, so six socks guarantee that two will be the same color.

Solution:

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

Solution:

2 cakes

At each toll bridge you are required to give half of your cakes, and you receive one back. Which leaves you with 2 cakes after every bridge.

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.