"What is 4.51 to 1 decimal place?" is the same as asking, "What is 4.51 rounded to one decimal point?" or "How to round 4.51 to one decimal place."

The main point (pun intended), is that you want to round 4.51 to one digit or place after the decimal point. Thus, we want to eliminate the last digit in 4.51. To eliminate the last digit, we use the following One Decimal Place rules to get the answer:

One Decimal Place Rule #1:

If the last digit in 4.51 is less than 5, then remove the last digit.

One Decimal Place Rule #2:

If the last digit in 4.51 is 5 or more and the second to the last digit in 4.51 is less than 9, then remove the last digit and add 1 to the second to the last digit.

One Decimal Place Rule #3:

If the last digit in 4.51 is 5 or more and the second to the last digit in 4.51 is 9, then remove the last digit, make the second to last digit 0, and add 1 to the number to the left of the decimal place.

When rounding 4.51 to one decimal place we use One Decimal Place Rule #1.

Therefore, the answer to "What is 4.51 rounded to 1 decimal place?" is:

4.5