I am trying to convert $10^{20}$ electron-volt to ton of TNT.
What I know is:
There are $10^{20}$ eVs which is 1 sig fig.
1 electron-volt = $1.602*10^{-19}$ J which is 4 sig figs.
1 ton of TNT = $4.18 * 10^9$ J which is 3 sig figs.
This is my calculation
$10^{20} * (1.602*10^{-19}) = 16.02J$ or $20J$ because of 1 significant digit
$\frac{20}{(4.18 * 10^9)} = 4.78469*10^{-9}$ ton of TNT or $5*10^{-9}$ ton of TNT with 1 significant digit.
I was told this is an incorrect answer. The other answer I tried involved determining the significant figures in the last step so:
$10^{20} * (1.602*10^{-19}) = 16.02J$ $\frac{16.02}{(4.18 * 10^9)} = 3.83254*10^{-9}$ ton of TNT or $4*10^{-9}$ ton of TNT with 1 significant digit.
I've tried $5*10^{-9}$, $4*10^{-9}$, and even $3.83*10^{-9}$ and they are all wrong. I have not been told the correct answer, so I am not sure what I did wrong. Did I make a mistake in my calculation or am I messing up the significant figures?