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Dew point is 20.

Explanation

Formula for relative humidity is given as

Where both actual vapor pressure and saturation vapor pressure are at same temperature.

Given

Temperature of air =20

Relative humidity=100%

So,

actual vapor pressure =saturation vapor pressure at the temperature 20

The temperature at which relative humidity is 100% i.e. air is completely saturated, is called dew point.

Here, the temperature at which the air is completely saturated is 20.

So, dew point is 20.