The temperature in the high-altitude desert of Ladakh is recorded as 4°C at noon. By midnight, it drops by 15°C. What is the midnight temperature? [Page No. 46]

Temperature at noon = 4°C
And by midnight, drop in temperature = 15°C
A "drop" means the temperature decreases, so we treat it as a negative change (-15°C).
Thus, midnight temperature = 4°C - 15°C
                                              = -11°C
Hence, the temperature falls below zero, so it becomes 11 degrees below freezing point.