How many grams of alloy which is 25% copper by weight must be mixed with 500 g of alloy which is 12% copper by weight to make a final alloy which is 20% copper

Question

help iv been stuck on this for so long!!!!!!!!!!!!!!!!!!

Answer 1

Two methods to solve this:

First:  (x g)(25%) + (500 g)(12%) = (500g + xg)(20%)

25x + 6000 = 10,000 + 20x

5x = 4000

x = 800 grams of the 25% alloy should be mixed with 500 g of the 12% alloy to make 1300 g of a 20% alloy.

 

Second method is called allegation (google it):

HAVE............WANT..........NEED.....

25%................................8 parts..

.....................20%........................

12%...............................5 parts..

 

Given that 5 parts = 500 g, each part = 100 g.  Since you need 8 parts of 25%, that equals 8x100 = 800 grams of 25%