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Q.5 The temperature at 12 noon was 10°C above zero. If it decreases at the rate of 2°C per hour until midnight, at what time would the temperature be 8°C below zero? What would be the temperature at midnight?
Answer :

Temperature at 12 noon was 10°C above zero i.e. +10°C
Rate of decrease in temperature per hour = 2°C
Number of hours from 12 noon to midnight = 12
Therefore Change in temperature in 12 hours = 12 × (-2°C) = -24°C Therefore Temperature at midnight = +10°C + (-24°C) = -14°C Hence, the required temperature at midnight =-14°C Difference in temperature between + 10°C and -8°C = +10°C – (-8°C) = +10°C + 8°C = 18°C Number of hours required = 18?C ÷ 2?C= 9 hours Therefore Time after 9 hours from 12 noon = 9 pm.