Determine the water temperature after mixing 100g of boiling water and 300g of water taken at a temperature of 20c.

Q = С * m * (tк – tн), С – beats. heat capacity of water, m – mass of water (boiling water), tc – end. pace. water, tn – early. pace. water.
The amount of heat given off by boiling water:
Q1 = C * m1 * (t1 – t3), m1 = 100 g = 0.1 kg, t1 = 100 ° C.
The amount of heat received by water:
Q2 = С * m2 * (t3 – t2), m2 = 300 g = 0.3 kg, t2 = 20 ° С.
С * m1 * (t1 – t3) = С * m2 * (t3 – t2).
m1 * (t1 – t3) = m2 * (t3 – t2).
0.1 * (100 – t3) = 0.3 * (t3 – 20).
10 – 0.1t3 = 0.3t3 – 6.
0.4t3 = 16.
t3 = 16 / 0.4 = 40 ° C.