difficult / hard, probably impossible

(number theory) negative number

to use / to employ / to make use of, to apply / to spend

calculation / forecast

to calculate

use one thing for many purposes / work someone hard

to use one's head

a machine used for performing calculations; a calculator / computer

spreadsheet - manual calculation of digits in the grids / (computing) spreadsheet - automatic calculation of digits in the grids by using computer.

calculating