One alloy is one part copper to three parts tin. A second alloy is one part copper to four parts tin. How much of a second alloy shold be combined with 24 pounds of the first allow to obtain a new alloy that is two parts copper to seven parts tin?

I made this chart:

Code:

Amount (lbs) % Copper Total Copper
Alloy 1 24 .25 .25(24)
Alloy 2 x .20 .20x
Mix 24+x 2/7 2/7(24+x)

And then got:

$\displaystyle .25(24) + .20x = .29(24+x)$

$\displaystyle 6 + .20x = 6.96 + .29x$

$\displaystyle -.96 = .09x$

$\displaystyle -10.67 = x$

Where did I go wrong?