Given :
The focal length of a concave mirror is 18 cm.
To Find :
The radius of curvature of the concave mirror.
Solution :
We know,

Therefore, the radius of curvature of concave mirror is 36 cm.
Hence, this is the required solution.
Answer:

Explanation:
First of all, let's convert from nanometres to metres, keeping in mind that

So we have:

Now we can convert from metres to centimetres, keeping in mind that

So, we find:

Answer : B) The cow pulls back on the girl.
From newton’s third law we know that every action has a reaction force pushing back. So when the girl pulls on a cow, the cow is pulling back on her.
The answer is: Salt! :)
Have a great day!