0.0136 g + 2.70 × 10-4 g - 4.21 × 10-3 g = ? how many digits to the right of the decimal point should be used to report the result?

You should use 5 digits to the right of the decimal point.

Explanation:

To be precise in the mathematical operation you have to conserve all significant digits .

Replace the scientific notation for its equivalent in decimal notation:

2.70 x 10-4 g = 0.00027  g

4.21 x 10-3  g = 0.00421  g

Then, compute the addition and subtraction :

0.01360  g

+  

0.00027  g

-  

0.00421  g

0.00966 g = 9.66 x 10-3 g

That means you should use 5 digits to the right of the decimal point, which are the biggest amount of digits necessary to represent  2 of the numbers involved in the mathematical operation.