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Question:

When 100g of a liquid A at 100°C is added to 50g of a liquid B at temperature 75°C, the temperature of the mixture becomes 90°C. The temperature of the mixture, if 100g of liquid A at 100°C is added to 50g of liquid B at 50°C, will be?

70°C

85°C

80°C

60°C

Solution:

100 × SA × (100 - 90) = 50 × SB × (90 - 75)
1000SA = 750SB
2SA = 1.5SB
SA = 3/4 SB
Now, 100 × SA × (100 - T) = 50 × SB × (T - 50)
2 × (3/4) × (100 - T) = (T - 50)
3/2 × (100 - T) = (T - 50)
300 - 3T = 2T - 100
400 = 5T
T = 80.