Answer:
51.515
Explanation:
When you round a number to three decimal places, not only you need to cut your decimals to the third one, but need to examine the following (fourth decimal in this case) to decide if you need to round using the digit in the third decimal that appears, or use the following digit.
The rule is, if the following decimal is any of the following digits: 5, 6, 7, 8, or 9, you use in the rounding place one digit ABOVE the one that appears .
In our case, 51.51488 you look at the fourth decimal, and since it is an "8", you change the third decimal "4" into a "5". Leaving the following rounding:
51.515,