The total number of marbles stays the same (12 out, 12 in).

[[model:questions/solutions/q5401.png]]

• In the end, ratio Red : Blue = 1 : 4 → Red = 1 unit, Blue = 4 units, Total = 5 units
• Given answer: total = 120 marbles
• 5 units = 120 → 1 unit = 24
• Red in the end = 24, Blue in the end = 96
• Red at first = 24 + 12 = 36
• Blue at first = 96 − 12 = 84
• Check initial ratio: 36 : 84 = 3 : 7
• Total at first = 36 + 84 = 120

There were 120 marbles in the box.
Check: After change, 24 : 96 = 1 : 4. ✓