A serving tray of brass weighs 21 oz. The brass is an alloy of 70 percent copper & 30 percent tin. What is the weight of the copper in the tray?

Expert Answers
kborgeson eNotes educator| Certified Educator

When you have an object that is a certain percent of one metal combined with a second metal we can see then how the two metals work together to make a whole brass tray. 

If the whole tray is 100%, and the problem tells you that out of this 100%, 70% is copper and 30% is tin.  With this information you can use your percents to find out how much of each metal is present in the entire tray.

Since the whole tray weighs 21 oz., you should ask yourself what is 70% of 21 oz.   The "of" in math almost always means to multiply.  

So if you take 70% of 21 oz. then really you need to multiply 21 by 70%, but it is not going to work unless you take 70% out of percent form and put it into decimal form. 

To take a percent from percent form to decimal form you need to divide it by 100, since percent is ALWAYS out of 100. 

So then your problem will look like this: .7 x 21

The answer is: The tray is made up of 14.7 oz. of copper.

If you wanted to find out how much tin is in the tray you would say 30% of 21 oz. or .3 x 21. 

Hope this helps.  Check out the websites below for other tips on percents.  :)

cburr eNotes educator| Certified Educator

You know that the brass is 70% copper and 30% tin.  You also know the tray weighs 21 oz.  So, you just need to find 70% of that 21 oz weight.

Remember that 70% is the same as 70/100.  You can write that as a decimal:  0.70 (seventy one-hundredths).

To find 70% of 21 oz:  21 x .70 = 14.7 oz.

So, your answer is that the copper in the tray weighs 14.7 oz.