An alloy of copper is 10% copper and weighs 25 pounds. A second alloy is 18% copper. How much (to the nearest tenth lb.) of the second alloy must be added to the first alloy to get a 13% mixture.

let the weight of the second alloy be y

convert the percentage to fraction, that is 10/100=0.1, 18/100=0.18, 13/100=0.13

Total weight if the mixture is 0.1 x 25+0.18y=0.13 (25+y)

2.5+0.18y=3.25+0.13y

like terms together

0.18y + 0.13y = 3.25-2.5

0.05y=0.75

divide both side by 0.05

Y is therefore 15 pound of 18% copper is needed