Use graph paper stuck to cardboard, drawing the lines carefully as shown. The scale on the left and right number lines are twice that of the middle number line. Make sure that the middle number line is exactly half way between the two outside lines and the v-shaped notches (on the sides) line up exactly with the numbers. To add two numbers, for example 3+ 12 , take an elastic band and use it to connect 3 on the left to 12 on the right. Where the elastic band intersects the middle line, here at 15, is the answer.

The device appears extremely primitive but don’t be fooled. It can be used to add positive and negative numbers and numbers in a different base. For example in base 5 (see table on the right) , 3+13=21!