It is called " junk"
The number left over in a division problem is called the "remainder".
stray
Your question isn't fully clear, but if you are referring to the process of division of numbers, the "number left over" is called a remainder.
The left over fruit material is called pulp.
Those materials left over after the economically extractable portion of the mineral being mined has been separated from the ore.
Disposable income.
The remainder.
solifluction
Leftover material is called scrap.
The numbers left over from dividing a number are called the "remainder".
Remnants
A Surplus