1. how do you determine how many digits to keep in an answer obtained by multiplying or dividing

2. how is the number of significant figures determined for an answer obtained by adding or subtracting

The answer to your question is below

Explanation:

1.- The significant figures for multiplication or division is equal to the least number of significant figures in any number of the problem.

Example:    2.5  x  3.42 = 8.55

  2.5 has two significant figures

  3.42 has three significant figures

  The result must have 2 significant figures, for the example the answer is

                  2.5 x 3.42 = 8.6

2.- The significant figures for addition and subtraction the answer must be rounded to the least number of significant figures in the problem.

Example

                 1.453 + 82.2 = 83.653

Adjust to the least number of significant figures

                                      = 83.7

because 82.2 has only one significant figure after the decimal point.