In the given puzzle, the logic is the product of the number and its preceeding number gives the result.

9 = 9 x 8 = 72

8 = 8 x 7 = 56

7 = 7 x 6 = 42

6 = 6 x 5 = 30

5 = 5 x 4 = 20

Similarly,

3 = 3 x 2 = 6.

Plane and locomotive are synonyms to each other, Similarly, pilot and flight engineer are synonyms to each other. 

Hence, plane is to locomotive as pilot is to engineer.

Obsession is to compulsion as cognition is to behavior.

The number of letters in the given word RITUAL = 6

Then, 

Required number of different ways can the letters of the word 'RITUAL' be arranged = 6!

=> 6 x 5 x 4 x 3 x 2 x 1 = 720

Let the number of boys = b

Let the number of girls = g

From the given data,

81b + 83g = 81.8(b + g)

81.8b - 81b = 83g - 81.8g

0.8b = 1.2g

b/g = 1.2/0.8 = 12/8 = 3/2

=> g : b = 2 : 3

Hence, ratio between the number of girls to the number of boys = 2 : 3.

The given series is 6, 6, 12, 36, 144, 720, ?

6

6 x 1 = 6

6 x 2 = 12

12 x 3 = 36

36 x 4 = 144

144 x 5 = 720

720 x 6 = 4320

Hence, the next number in the given number series is 4320.

A chemical reaction in which involves the absorption of heat from the environment is called a Endothermic reaction. 

Examples : Dissolving ammonium chloride in water, photosynthesis, melting ice cubes, mixing of potassium chloride with water,...

In the contrast, chemical reactions involving release of heat are known as exothermic reactions.

Examples : Combustion, coal is burnt, mix of CaO with water,...