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**HallsofIvy** I understand what you mean but if you were my student, I would mark you down for "let x and y represent the two alloys". x and y are numbers, not "alloys". What you **mean** is "let x and y be the weights of the two alloys in kilograms".

"There were two different copper alloys of total weight 50 kg"

So x+ y= 50.

"The first contains 40% less copper than the second. Determine the percentage of copper in the first and second alloys, if it is known that there were 6kg of copper in the first alloy and 12 kg in the second."

So if we take "p" to be the percent of copper in the first alloy, 6/x= p. Since the second had 40% more copper than the first, it has p+ .4 percent copper: 12/y= p+ .4. That gives you three equations to solve for x, y, and p