Two alloys of iron have different percentages of iron in them. the first of them weights 6 kg and the second one weights 12 kg. one piece of equal weight was cut off from both alloys. the peice from the first was mixed with the leftover of the second alloy and the peice from the second alloy with the leftover part of the first alloy. as a result the percentage of iron became same in both the alloys. what was the weight of each piece of cut off?