Let's look at the rules for adding and subtracting numbers.
120 + 1 + 1.5 + .5 = ?
First, let's look at the decimal places. How many decimal places do we know the answer to? Remember, you only know as much as your LEAST ACCURATE piece of data. So look for the number with the fewest decimal places.
120 and 1 both have no decimal places. So your answer will have no decimal places.
Now look to the left of the numbers. Whichever number has its last significant figure furthest to the left determines the last significant figure you should report in your answer.
Relax! It's not rocket surgery.
It's just a question of rounding your answer to the same level of accuracy as your worst piece of data.
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