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Say you have 5 rings, each weighing 18 grams (10% alloy and 90% gold).

If you would melt them all and add silver to make it 75% gold,

1) how much silver do you have to add?

2) Does melting 5 rings with 18grams each give 90 grams in total?

3) is this equation to solve for this problem right?

let x = amount of silver to add

total grams = 90 g

total grams (.90) + x silver (100%) = total grams + x (.75)

90(.9) + 1x = 90(.75)

81 + x = 67.5 + .75x

x = -54

4) Am I doing/thinking them all wrong?

thanks.