Math Help Needed!!

[touchher]

I don't understand how to do these problems/ set it up. Any help would be greatly appreciated. Answer is 18g.

a jeweler has five rings, each weighing 18 g, made of an alloy of 10% silver and 90% gold. he decided to melt down the rings and add enough silver to reduce the gold content to 75%. how much silve should he add?

 

[swiftaw]

Okay, 5 rings each 18g, so total weight is 5*18 = 90g

Now, they are 10% silver and 90% gold, so 9g of silver and 81g of gold.

Want to add silver, until gold content is 75%. Thus, let's suppose the amount needed to be added is x.

So then there will be (9+x)g of silver, 81g of gold for a total weight of (90+x)g

Gold needs to be 75% of the total, so we know that 81/(90+x) = 0.75

Re-arranging the final equation, gives us 81 = 0.75*(90+x)
Therefore 81 = 67.5 + 0.75x, or 0.75x = 13.5, or x = 18g

So 18g of silver needs to be added

 

[touchher]

thank you!!

 

[nakamuramori2]

woo... cant think anything... is this for GMAT math??

 

[Schtumple]

Okay, 5 rings each 18g, so total weight is 5*18 = 90g

Now, they are 10% silver and 90% gold, so 9g of silver and 81g of gold.

Want to add silver, until gold content is 75%. Thus, let's suppose the amount needed to be added is x.

So then there will be (9+x)g of silver, 81g of gold for a total weight of (90+x)g

Gold needs to be 75% of the total, so we know that 81/(90+x) = 0.75

Re-arranging the final equation, gives us 81 = 0.75*(90+x)
Therefore 81 = 67.5 + 0.75x, or 0.75x = 13.5, or x = 18g

So 18g of silver needs to be added

Are you superman??? How in gods name did you work that out????

 

[BigHungry04]

Logic. Pure Simple Logic.

 

[ChrisA]

Are you superman??? How in gods name did you work that out????

Come on now, this is 8th grade level work, introductory algebra.
Now maybe we can all complain how basic math and science education has gone down hill.