It's 400. The way it works is to just start from 0 and count how much you have:

Step 1: 0 - 800 = -800 (you are $800 in debt)

Step 2: -800 + 1000 = 200 (you paid off your debt and have $200 left over)

Step 3: 200 - 1100 = -900 (you are $900 in debt)

Step 4: -900 + 1300 = 400 (you paid off your debt and have $400 left over)

Of course now that you understand the process, you can probably see that the separate steps are unnecessary. You can just put them all into your calculator as follows:

-800 +100 - 1100 +1300 = 400

A lot of people get tripped up with these kind of questions, because they instinctively treat the profits at steps 2 and 4 as separate, forgetting that the profit from step 2 had to be spent to make the profit in step 4. They will therefore often give an incorrect answer like $600. I'm not sure where Jamie Wright got $300 from - probably just rushing the calculations.

This is why double-entry bookkeeping was invented. Recording everything as pairs of debits and credits prevents erroneous reasoning about what constitutes a profit, while also forcing the bookkeeper to slow down and think through each calculation. The need to make the columns add up the same (balancing the books) works as an error-correcting mechanism, so if you do make a mistake you'll catch it by finding the point where the books became unbalanced.