Questions

A Clock has 3 hands i.e, an Hour hand, a Minute hand and a Seconds hand.

The structural unit of compact bone is Osteons or Haverisan systems.

A biosphere is the sum of all the ecosystems established on planet Earth.

An ecosystem is an area in which the plants and animals all interact to create a living environment. Therefore ecosystem is a smaller unit of earth, all ecosystems together form the earth.

Mg(NO3)2 is Magnesium nitrate.

In the case of Mg(NO3)2, the metal in the compound is magnesium (Mg), and the non-metal here is called the nitrate ion (NO3). This makes the name of the compound simply magnesium nitrate.

You will get the answer in the riddle itself, if you read the riddle carefully.

The riddle says "A cowboy arrives in town on Friday." So Friday is the name of the horse on which he comes. Hence, the only possibility is he arrives on the horse friday on Tuesday, stays for 3 days and leaves on the same horse on Friday.