Relative humidity (RH) is defined as the ratio of the current amount of water vapor in the air to the amount of water vapor required for saturation at the same temperature, expressed as a percentage.
In this case, the air in the room has 15 grams of water vapor per cubic meter, and the saturation level is 20 grams of water vapor per cubic meter.
To calculate the relative humidity, use the following formula:
\[ \text{Relative Humidity (RH)} = \left( \frac{\text{Actual Water Vapor}}{\text{Saturation Water Vapor}} \right) \times 100 \]
Substituting the values:
\[ \text{RH} = \left( \frac{15 , \text{g/m}^3}{20 , \text{g/m}^3} \right) \times 100 = 0.75 \times 100 = 75% \]
So, the relative humidity in the room is 75%.