In science, an unknown value is represented by a letter in the Roman or Greek alphabet. They are used most often in physics, where equations are used to describe the relationship between physical properties. For example, in the equation formula_4, the letter "formula_5" represents an unknown (in this case, mass), and the letter "formula_6" also represents an unknown (in this case the amount of energy). If we know the value of all but one of the unknowns, then we would be able to easily find out the value of the last unknown—by "solving" the equation in terms of that last unknown.