It is useful to be able to do these sums in your head,or a piece of paper.However,for thousands of years people have used other things to help them count and calculate.The oldest of these is the abacus.This is an instrument in which you move beads along rods or wires in a frame.One of the most recent is the calculator.Some lazy people use a calculator to do dums they should be able to do in their head.They think it is the best way of avoiding mistakes.But you won't get the right answer on a calculator if you press the wrong buttons!