What is the value of this addition 2.55 mL + 45.723 mL + 0.30 mL?

48.57 mL

When measurements are added or subtracted, the answer can contain no more decimal places than the least accurate measurement. Thus, look at each number you are adding/subtracting and see how digits are to the right of the decimal place. The answer will have the same number of digits to the right of the decimal place as the smallest number of decimal places of the number you are adding. In this case, we are adding 2.55 (2 to the right), 45.723 (3 to the right), and 0.30 (2 to the right). The smallest number of places to the right is 2 digits; thus, the answer must be rounded to 2 decimal places.