Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain, try to get an upper bound on the quantity):
(d) the number of strands of hair on your head
The quantity used as a standard of measurement is called unit.
Example: Unit of time is second.
Define physical quantity.