The correct answer is (b) 25min
The explanation is: Average temperature of liquid in the first case = 60℃.
Rate of fall of temperature = 65-55 / 5 = 2°C /min.
Which is also equal to k(60 – 35) = 25k, where k is a constant.
∴ 25k = 2 Or k = 2/25
Average temperature in the second case = 40°C
Rate of fall of temp = 45 – 35 / t = k(40-35)
∴ t = 25mins.