Solution:

7

Explanation:
Put batteries into 3 groups: two groups of 3 and one group of 2.
It guarantees at least one of the group has got two working battery
Group of 3 requires 3 attempts to verify if 2 batteries of the group are working (B1+B2, B2+B3, and B1+B3) while the group of 2 requires one attempt (B1+B2)
Total attempts = 3 + 3 + 1 = 7

Solution:

797

Explanation:
797 is a prime number
79 is a prime number
7 is a prime number

Solution:

8 Steps

Explanation:
Steps------2L Gallon-------5L Gallon
A-------------12-------------0
B-------------07-------------5
C-------------07-------------0
D-------------02-------------5
E-------------02-------------0
F-------------00-------------2
G-------------12-------------2
H-------------09-------------5

Solution:

It can be done as shown below.

Solution:

As shown in solution picture, flip the first number and stack on the second number, you will get fish eight and triangle respectively.

Solution:

This type of puzzle is referred to as an informal fallacy puzzle. The general idea is that the logic that is presented to the user appears at first glance to be correct, but in actuality has an error in it.
Actually,
Total pound spend by elder brothers = 9+9 = 18
=>17spend on shoes + 1pound (Kylian kept for himself)

Solution:

10.

Explanation:
Most of the people will give the answer as 6 or 3.
But the total number of holes are ten.
> 1 hole for the neck.
> 2 holes for the arms.
> 1 hole for the torso.
> 3 torn holes in the front.
> 3 torn holes in the back.

Solution:

-> Pour the water in the barrel.
-> Freeze it.
-> Pour the milk in the barrel.

Solution:

The death was ruled a suicide.

Solution:

Pick 1st.

Explanation:
> Take 1 coin
> Pick the opposite number of coins from the other soldier i.e if other soldier pick 1 coin, you pick 2 and if he picks 2 coins you pick one
> After 15 moves, coins remaining will be 23 - 1 - 7(2+1) = 1
> The other solider need to pick the last and he will loose.