The correct answer to this question is E, 1.56%. To get this answer, we can divide the given values in the word problem. We will divide the 28 thousand gallons of filter water by the 1.8 million values of backwashing. This will provide us with 0.0155555556. This would be the correct answer, except the question is asking for a percent.
To get the percent, we would need to move the decimal point in the value two spaces to the right. This gives us 1.55. We don't see 1.55% as one of the multiple choice answers, so we would round up the second 5 to 6, being that the following numbers are more significant. This will give us an answer of 1.56%.