Based on the information given, it should be noted that the wet-bulb temperature will be 16°C.
A psychrometer simply means a hygrometer that has two thermometers. In this case, one is hot while the other is moist and ventilated.
From the complete information, when the dewpoint is 15°c, the wet-bulb temperature on a sling psychrometer if the dry-bulb temperature is 18°c is 16°C.
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