Answer: 2,142 IU
Explanation:
We have 1428 IU of calcium in a liter of orange juice.
We know that the ratio of calcium in orange juice to calcium in milk is 4 to 6.
So, 1428/4 = 357.
Then, to get the amount for milk, we have 357*6 = 2142.
So the answer is 2142 IU of calcium in a liter of milk.