I understand that
1) digits from 1-9 are significant
2) Zeros between two other significant digits are always significant
3)One or more additional zeros to the right of both the decimal place and another significant digit are significant.
4)Zeros used solely for spacing the decimal point (placeholders) are not significant.

But, adding them confuses me. There are no "zeros" in these equations, and my atronomy professor told us "not to solve these like your math teacher would". So, idk what to do.

3) For the product (2.3456x10^4)(1.2x10^9)
which result has the correct number of
significant digits?
a) 2.8x10^13
b) 2.81x10^13
c) 2.815x10^13
d) 2.8147x10^13
e) 2.81472x10^13


4) Assuming each of the following numbers
has the correct number of significant digits as
written, for the sum 23.456 + 1.2, which result
has the correct number of significant digits?
a) 20
b) 25
c) 24.7
d) 24.66
e) 24.65