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Official Explanation

Since the ratio is \(1:2:3:4,\) we can think of the jars containing \(1x, 2x, 3x,\) and \(4x\) marbles for some whole number, \(x.\) Then \(1x + 2x + 3x + 4x = 60,\) so \(10x = 60,\) and \(x = 6.\) This makes the current count of marbles in the jars 1(6), 2(6), 3(6), and 4(6), or 6, 12, 18, and 24. If each jar were to contain 60 ÷ 4 = 15 marbles, the ratio would become 15 : 15 : 15 : 15, which reduces to 1:1:1:1, as required. The minimum number of stones that must be moved to effect this change is 3 + 9 = 12, because three stones moved from the 18 jar to the 12 jar would leave those jars with 15 stones each, and nine stones moved from the 24 jar to the 6 jar would give those jars 15 each.

Answer: 12