Two bags contain marbles in the ratio 5 : 7. If 12 marbles are transferred from the second bag to the first bag, the ratio of marbles in the two bags becomes 7 : 5. Find the original number of marbles in the second bag.
42
Let the original numbers of marbles be \(5x\) in the first bag and \(7x\) in the second bag.
After transferring 12 marbles from the second to the first, the counts become \(5x + 12\) and \(7x - 12\), and their ratio is 7 : 5.
Set up the equation \(\frac{5x + 12}{7x - 12} = \frac{7}{5}\) and cross-multiply: \(5(5x + 12) = 7(7x - 12)\).
Expand: \(25x + 60 = 49x - 84\), so \(144 = 24x\) and \(x = 6\).
The original number in the second bag is \(7x = 7 \times 6 = 42\).
Hence, the second bag originally had 42 marbles.
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