Question

The boiling point of a sample is 15°C, bit this sample was observed to have 18°C at atmospheric pressure
answer immediately

Answer 1

Boiling point depends of pressure, impurities in the sample. Actually fluctuation in atmospheric pressure should not change the boiling point so much, most probably your sample has impurities. Also possibility of using faulty thermometer can't be ruled out. May be there is some offset in your thermometer.