What rules should you follow when adding and subtracting? check all that apply. 1. the exponents in scientific notation should be added or subtracted. 2. the exponents need to be the same so the leading numbers can be added or subtracted. 3. the result has the same number of decimal places as the least precise number. 4. the result has the same number of significant figures as number with the fewest significant figures.

The answer to this question would be: 
2. The exponents need to be the same so the leading numbers can be added or subtracted.
3. The result has the same number of decimal places as the least precise number.

The exponent is the 10^x that used in scientific notation. .When adding 12.3* 10^5 with 34.56 * 10^4, you need to make the exponent same first. In this case, it would become 12.3* 10^5 + 3.456 * 10^5= 15.756* 10^5
The decimal place could be rounded up to the least precise number. I'm not sure with the significant figures but it seems to use the biggest one.