Answer:
12
Explanation:
this equation can be derived from the question
55 - 4x < 10
< = less than
x = number of days
solve for x
55 - 10 < 4x
45 < 4x
x < 11.25
at the 11th day, there would be 11 keys left
on the 12th day, there would be 7 keys left
the correct answer is 12 days