how much cold water is needed to have a mixture of warm water that has a temp of 45 deg cel and mass of 4000g?

Question

the temp of the water that you have is 20 deg cel and the other 100 deg cel.

Answer 1

The weighted average temp must be 45 degrees
The total mass is 4000 grams.

Turn that into algebra:

20 deg x masscold + 100 deg x masshot = 45 deg x 4000g
masscold + masshot = 4000g

2 equations, 2 unknowns, you can solve the algebra from there.

Answer 2

20x + 100 (1-x) = 45

solve for x = 0.8125

Mx = 0.8125 * 4000 = 3250 g of cold water