Enry lives in mississippi, which has a sales tax of 7%. he just bought a bed whose full price was $1600, but he got 30% off, because the store was having a sale. what was the total amount that henry paid?

I wish Enry would have looked at the tag on the bed. In most places full price means including the sales tax. However I don't think you mean that. I think that was your before tax amount.
So take 30% off the original price.

There are two ways you can do that. You can take 30% of 1600 and subtract that. We'll do that now.
1600 * 30/100 = 480 dollars. Now take that off 1600
1600 - 480 = 1120

Or you can take 70% of 1600 which should give you 1120 right away.
1600 * 70/100 = 1120

Now you can add on the sales tax and again, there are two ways to do it, but I'm only going to do it one way. The quick way to add on a sales tax (or any tax) is add the % as a fraction on to 1. 1 + 7/100 = 1 + 0.07 = 1.07

1120 * 1.07 = $1198.40

You could do this the other way. 7/100 * 1120 = 78.40.
1120 + 78.40 = 1198.40  

The answer should be b.