Blackboard test. Various solutions to sums written around Einstein's famous mass-energy equation, E=mc2, for a school test. Einstein worked out the interrelationship of mass and energy in his special theory of relativity of 1909. In the equation, E is energy, m is mass, and c is the velocity of light. Since the velocity of light is such a large quantity, a small amount of mass multiplied by the square of the velocity of light is equivalent to a huge amount of energy.