#### Answer

$a. 52.9$
$b. 52.86$

#### Work Step by Step

Step 1: Locate the digit to the right of the given place value.
Step 2: If this digit is 5 or greater, add 1 to the digit in the given place value and drop all digits to its right.
If this digit is less than 5, drop all digits to the right of the given place.
$a.$ In $52.8647$ 6 is greater than 5, so we have to add 1 to 8, and drop all digits to its right. The answer is $52.9$
$b.$ In $52.8647$ 4 is less than 5, so we have to drop all digits to its right. The answer is $52.86$