Subtracting whole numbers does not include subtracting negative numbers, whereas when subtracting integers you can include negative numbers.
If you would like to know how many hours will Lena need to work before she can afford to buy the computer, you can calculate this using the following steps:
$900 - $473 = $427
$427 / $7 = 61 hours of yard work
Result: Lena will need to work 61 hours before she can afford to buy the computer.
You should ender 0.5 for a half note, 0.25 for a quater note, 0.125 for a eigth note, 0.0625 for a sixteenth note.
I believe the answer would be B, good luck