Hello guys, I hope this question is not too juvenile for the people on here...here it goes(taken from my economics class)

If your income increases from $10,000 per year to $14,000 per year and your tax payment increases from $2,000 to $2,840, the marginal tax rate is:

a)20%
b)21%
c)25%

I answered 20% and got it marked wrong...is that correct? Any ideas?

Thanks
Brad