no, that answer is not *only* half correct. that answer is 100% correct. The question is how long to cover half the lake. The answer for covering half the lake is the entire correct answer.
Which half? Is this an unusual lake whose area does not have two halves?
One half of its area, the inner portion, is covered in 47 days. The other half of its area, the outer portion, is covered in one day, the 48th day.