the mass of a quarter is, if turned into pure energy, equal to 121 kilotons of TNT

E=m*c*c [J] m:mass[kg] c:light velocity (approx 300,000,000[m/s]) 1[TNT kT] is approx 4,184[TJ]

I'm sorry! 4,184[TJ] is wrong. 4,184[GJ] is correct.