Whats a constant and a variable and a coeffecient ?

A constant is a number, or a letter that represents a number that
never changes.  From the time the problem starts until as long
as it lasts, its value is constant.

A variable is a letter that stands for a number that can change.

A coefficient is each of the items in a group of them that are all
multiplied together.

Example:  17x + 4y

That's an expression with two terms.
The terms are  '17x'  and  '4yz'.

In the first term:
'17' is the numerical coefficient.
'x' is the literal coefficient.

In the second term:
'4' is the numerical coefficient.
'y' and 'z' are both literal coefficients.

20000 as first it increases by 2500 then by 5000 so next it will increase by 10000