mixture problem?

Question

How many kilograms of brass containing 63% copper by weight must be melted with 1140 kg. of brass containing 72% copper to produce brass containing 67% copper.

Answer 1

Set up two equations:
Let x = weight of brass with 63% copper
Let y = total weight after melting the mix
So: x + 1140 = y or x = y - 1140
Also: .72 (1140) + .63 (x) = .67 (y)
Substituting:
.72 (1140) + .63 (y-1140) = .67 (y)
820.8 + .63y - 718.2 = .67y
102.6 = .04y
y = 2565 kg of total mix

x = y - 1140
x = 2565 - 1140 = 1425 kg of brass with 63% copper is needed

Answer 2

Let x kg of brass containing 63% copper by weight that must be melted.
x*63/100 +72*1140/100 = 67*(x+1140)/100

x*63 +72*1140 = 67*x+67*1140

5*1140 = 4*x

x = 285*5

x = 1425 kg

1425 kilograms of brass containing 63% copper by weight must be melted with 1140 kg. of brass containing 72% copper to produce brass containing 67% copper.