Given

Temperature of open room = 20°C

Dew point =10°C

New temperature of room =15 °C

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

So, dew point will not change until the temperature of room is less than dew point.

In our question, room temperature drops to 15 which greater than dew point.

So, the new dew point will be same as the old one.