Proving once again that just giving answers away doesn't help people to learn. (I so revel in it when I get to say that.

) A lesson for those of you who give away answers. (Especially when they're wrong.)
This is a basic adjustment we use for doing cash flows, though likely it's an earlier chapter and they think they're getting you to understand the concept of accrued and deferred accounts. I think it's a bit early to learn this, but....
The $12,700 is correct. Something goes into receivables when you record the revenue but do not collect on it, right? i.e. you have Fees Earned to record, but you stick it into A/R because you haven't gotten paid yet. Same thing here, except it's interest. It makes no difference that it's interest. You record interest revenue that is earned, and put it into an interest receivable because it has not been collected yet.
This balance in the receivable went up from 2005 to 2006. (Assuming these are year-end balances, and I can't see anything else they could be.) If something goes into a receivable because it hasn't been collected, that balance that went up is because $400 of the interest revenue never got collected. It shows the revenue as $13,100, which was recorded and shows on the income statement... but $400 never collected. So you collected 13,100 LESS the 400 = 12,700 in cash collected.
All current assets will do the same thing. The adjustment you want to make goes the opposite direction of what the balance did. Balance went up, so we subtract to make the adjustment off the revenue. The 10,825 is incorrect because it only accounts for the beginning balance, and the 8950 subtracted both balances instead of the
difference in the balances.