A unit is what one can use to indicate the measurement of a quantity. For example, a unit of length could be an inch or a metre or a mile, etc. In order to ensure that readers all have a common standard, an international system of units exists known as the SI system. There are seven basic SI units from which all other units are derived. Algebra is a means of solving mathematical problems using letters or symbols to represent unknown quantities. The same laws apply to algebraic symbols as to real numbers. The laws of indices also apply to algebraic symbols. A formula or equation is an algebraic means of showing how a law or rule is applied. The equals sign in a formula or equation is similar to the pivot point on a pair of scales. Formulae may be rearranged (transposed) such that any symbol can be shown in terms of the other symbols.