SOLUTION: A certain alloy is 30% copper, 45% zinc and 25% iron. By adding 25 grams of zinc to it, the new alloy now has 56% zinc. How much copper is in the alloy?

Instead of doing your homework for you, I'll do a different problem exactly like
yours step-by-step which you can use as a model to do yours by.

Let x = the total number of grams of ORIGINAL alloy.
Then 
0.26x = the number of grams of copper.
0.46x = the number of grams of zinc.
0.28x = the number of grams of iron.

We start with 0.46x grams of zinc and add 40 grams of zinc to that. That gives
us 0.46x + 40.  That increases the amount from x grams to x+40 grams. So adding
that zinc caused there to now be 55% of that increased amount of x+40.

0.46x + 40 = 0.55(x+40)

We can remove the decimals by multiplying through by 100.

46x + 4000 = 55(x+40)
46x + 4000 = 55x + 2200
     -9x = -1800
       x = -1800/(-9)
       x = 200

So the amount of original alloy was 200 grams.  So

0.26x = the number of grams of copper = 0.26(200) = 52 grams
0.46x = the number of grams of zinc = 0.46(200) = 92 grams
0.28x = the number of grams of iron = 0.28(200) = 56 grams

Now do yours exactly the same way, step-by-step.
 
Edwin