How much 20% and 40% copper alloy should be mixed to yield 500 kilograms of 25% copper alloy

Answer 1

box 1
x kg
.2x copper
.8 x whatever

box 2
y kg
.4 y copper
.6 y whatever

box 3
x+y kg = 500
.25*500 = 125 kg copper
375 kg whatever

so
125 = .2 x + .4 y
500 = x + y

take it from there