You can solve this problem algebraically but if you use intuition a little it'll be much faster.

First of all, we know this question is structured to have "replacement". That basically means we put marbles back in the bag once we draw them. This means the probability of drawing one red marble (then putting it back) and drawing another red marble is the same.

With that said, we know the probability of drawing 2 NON-RED marbles is 16/25. This means drawing 1 NON-RED marble is 4/5. Then we know the probability of drawing a RED marble is 1-4/5 = 1/5.

Then by definition, Prob(red marble) = #RED/#TOTAL = 1/5 Thus 12/Total = 1/5 => Total = 60!

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