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Earlier discussion
1 older comment kept for context.
David Ajayi
Explanation
Heat lost by water at 100°c= heat gained by water at 25°c + heat gained by calorimeter
mc∆T = mc∆T + C∆T
m x 4.18x (100-50)= 20x 4.18x(50-25) + 80 x (50-25)
209m= 2090 + 2000
209m = 4090
m= 4090/209= 19.57= 20g (C)