I need someone to explain the workout of this question please

**Question** A jar contains 10 red marbles and 30 green ones. How many red marbles must be added to the jar so that 60% of the marbles will be red?

**Answer** Assuming the required number of red marbles to be=x

40+x marbles are in the jar, of which 10+x are red. Since we want the fraction of red marbles to be 60%, we have:

__10+x__ = 60% = 3/5

40+x

Cross multiplying we get:

50+5x=120+3x => 2x=70 => x=35

**I am having problem understanding why the question is done this way? As in, I dont understand why 10+x is divided by 40+x ??? Is there a formula applied in it? If so i'm not aware of it, im not familiar with the formula, please explain. **

And if not i cant understand the logic of how/why is it worked out this way? Is there any other way of doing it? Some simple to understand, straight forward way?