Hi,
The final mixture is to be 200kg od 37% salt solution by mass. Thus 37% of the 200 kg is salt. 37% of 200 is
^{37}/_{100} 200 = 74 kg.
This is made up of some salt from the 30% solution and some from the 40% solution. Suppose that you use x kg of the 30% solution and the remainder, 200  x kg, from the 40% solution. 30% of the x km from the 30% solution is salt, so the amount of salt contributed to the final mixture from the 30% solution is
^{30}/_{100} x kg.
40% of the 200  x km from the 40% solution is salt, so the amount of salt contributed to the final mixture from the 40% solution is
^{40}/_{100} (200  x) kg.
The sum of these two is 74 kg.
Penny
