To round a number, you first have to determine what you are rounding to. in this case, it's the nearest dollar. Then, you determine whether to "round up," to the next higher number, or "round down," which is to stay at the number it's currently at, without the numbers following what you're rounding to. If the number after the place you're rounding to is equal to or higher than 5, you'll round up. If it's lower, you'll round down.
So, for your questions, the answers would look like this:
$8.98 rounds to $9.00, because 9 is bigger than 5.
$6.49 would round down to $6.00, because 4 is less than 5.
$2.51 rounds up to $3.00, because 5 is equal to 5.
$9.78 would round up to $10.00, because 7 is higher than 5.
$39.89 will round up to $40.00, because 8 is bigger than 5.
