L(x, y, k) = x^0.4·y^0.6 - k·(2·x + 4·y - 60)
L'x(x, y, k) = 2·y^(3/5)/(5·x^(3/5)) - 2·k = 0 --> k = y^(3/5)/(5·x^(3/5))
L'y(x, y, k) = 3·x^(2/5)/(5·y^(2/5)) - 4·k = 0 --> k = 3·x^(2/5)/(20·y^(2/5))
y^(3/5)/(5·x^(3/5)) = 3·x^(2/5)/(20·y^(2/5)) --> y = 3/4·x
NB
2·x + 4·y - 60 = 0
2·x + 4·3/4·x - 60 = 0 --> x = 12