I am not 100% this is correct but this should be it. Hope this helps!
5) $4,000 is to low to get any commission meaning we must put it at 0
6) For $8,000, we have enough to start getting commission on our first $5,000. However, $8,000 doesn't match up to $10,000 or $15,000 so we have to leave the two rows with a 0 and total commission is $250 because our only commission is $250.
7) $18,000 lets us fill in all rows, first ill fill in the first 2 rows, $5,000 * 5% = $250. In addition, $10,000 * 8% = 800. So right now we have $1,050. However we need to get commission for anything over $18,000, so I do $18,000 - $15,000 = $3,000. Now to find the commission, $3,000 * 10% = $300. Now we found the commission, we need to add it all up to find the total. $1,050 + $300 = $1,350.
8) We already know our first two rows is $1,050, so lets not waste time and start the third. $27,500 - $15,000 = $12,500. Now, $12,500 * 10% = $1,250. Lastly, $1,050 + $1,250 = $2,300