Answer:
23rd hours
Explanation:
This is problem based on arithmetic progression in which our first term is 45 , and the common difference is -2
we have to find the n when the nth term is 2.
The formula for nth term of an AP is

replacing the values in the formula

hence at 22.5 hours it will reach 2F or we can say that after 23 hours the temperature will be below 2F