In the book in question 29, the answer is reported upto 2 significant digits while answer of   q 30 is reported upto 3 significant digits. why? how can one determine the number of significant digits to report to in these two questions? 

Dear Student
For addition and subtraction use the following rules:
 

1. Count the number of significant figures in the decimal portion ONLY of each number in the problem
2. Add or subtract in the normal fashion
3. Your final answer may have no more significant figures to the right of the decimal than the LEAST number of significant figures in any number in the problem.

For multiplication and division use the following rule:

The LEAST number of significant figures in any number of the problem determines the number of significant figures in the answer. (You are now looking at the entire number, not just the decimal portion)

Here second rule is applicable.

So If you see first question there are 2 significant digits in number 5.6

Similarly you can check for other question while solving.

Regards