A circle has a diameter of 10 centimetres. What is its circumference? Give the answer in terms of π.

Erklärung

The way round a circle is π multiplied by the width right across it, and that width is given directly here, so nothing needs halving or doubling first. Using 5 treats the given width as though it were the distance from the centre outwards, which shrinks the circle to half its stated size.