A constant-quantity is a constant value of some physical-quantity, like 3 meters or 55 miles per hour.
Constant quantities are distinguished from function-quantities, which map some quantities to other quantities.
There are two quantities in the above situation: age and the number of fingers on a person's hand. The number of fingers remains the same, irrespective of a person's age. So we say the number of fingers is a constant quantity.
Because, mass is the amount of matter contained in a given object.
What is mass?
Mass can be explained as the amount of matter a body contains. This is a constant quantity and will not change when the body changes its position or location.
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