Sorry I meant that when I use E=γmc^2 (total energy formula), I get the correct answer. E=(γ-1)mc^2 (kinetic energy formula) gives the wrong answer.
The question should be 20 GeV of total energy, right?
The answer is 3.079*10^-26 kg
I tried doing E=mc^2
20*10^9*1.602*10^-19=m*(3*10^8)^2
m=3.56*10^-26 kg
However when I do E=γmc^2 I get the correct answer.
Is the question wrong, because isn't it 20 GeV of kinetic energy, not total energy?