Explanation:
2 weeks = 14 days.
on 11 days of these 14 he had fever.
that means on 14-11 = 3 days he did not have a fever.
on 9 of these 11 fever days he had the fever in the evening.
and we know, when he had fever in the evening, he had no fever in the morning.
so, the number of days without fever in the morning is :
9 + 3 = 12
as he had no fever in the morning when he had a fever in the evening, AND when he did not have any fever at all ...