Math problem help!

[orangepopsicle]

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The question is: There are 10 red marbles and 30 green marbles in a bag, how many red marbles need to be added to change the percentage of red marbles in the bag to 60%?

I set up the equation as 10+x/30+10 = 60% but the answer is 35. Can somebody explain how to set up this equation? I am stumped.

 

[Lnguyen7]

%R = (#R/Total marbles)*100%
let x be the number of R must be added
60% = [(10 + x)/(30 + 10 +x)]*100%
Solving for x = 35

 

[alexis2010]

You forgot to add x to the denominator
Lnguyen7 is correct.