When measuring dimensions like the width of a doorway, the greatest possible error is typically half of the smallest unit of measurement used.
If we assume you measured the width to two decimal places (1.20 m), the smallest unit of measurement in this case is 0.01 m (or 1 cm). Therefore, the greatest possible error would be half of that:
\[ \text{Greatest possible error} = \frac{0.01 , \text{m}}{2} = 0.005 , \text{m} = 5 , \text{mm} \]
So, the greatest possible error of your measurement is ±0.005 m (or ±5 mm).