you have 25 lb of alloy which is 10% Cu, so you have 2.5 lb of Cu.

Now add x lb of the 18% alloy, this contains 0.18x lb of Cu, so the

new percentage of Cu is:

(2.5+0.18x)/(25+x)

which is the mass of Cu divided by the total mass, and we want this

to be 13% or 0.13, so:

(2.5+0.18x)/(25+x) = 0.13,

so:

2.5 + 0.18x = 0.13(25+x)

or:

0.05x = 0.13*25 - 2.5 = 0.75,

so:

x = 0.75/0.05 = 15 lb.

RonL