Question 190406

Let A=amount of alloy that we have to start with (I think you left this out)
and Let x=amount of tin that needs to be added

Now we know that the amount of copper before we add tin(0.84A) has to equal the amount of copper after we add tin ((0.82(A+x)).  So, our equation to solve is:

0.84A=0.82(A+x)  get rid of parens
0.84A=0.82A+0.82x  subtract 0.82A from each side
0.84A-0.82A=0.82x
0.82x=0.02A  divide each side by 0.82
x=0.024A

As you can see here, the amount of tin that needs to be added will depend upon the original amount of the alloy and you did not include that number.

Hope this helps---ptaylor