Questions

Vapour of carbon tetra chloride ccl4 is non-inflammable and is heavier than air. Hence it is used as a fire extinguisher under the trade name of pyrene.

Let the value to be find be 'x'.

 
Sum of individual column value = 14

 
C1 = 7 + 2 + 1 + 4 = 14
C2 = 3 + 8 + 1 + 2 = 14
C3 = 6 + 5 + 2 + 1 = 14

C4 = 2 + 4 + 4 + x = 14

x = 14 - 10

x = 4

Hence, the missing number is '4'.

Owl

France joined the Triple Entente because it needed protection from Germany.

France felt very heavily the aggression from Germany (a country which it shared a border with), especially over the tension Alsace and Lorraine and the constant switching over of these two counties.

It was also worried about the rapid military advancement in Germany and France wanted to ally itself with Russia and Great Britain.

In this position, the head light will be at the principal focus so that the rays of light falling on the cancave mirror will be rendered parallel