Assuming the inteest is annual, since it's simple that means we just have to analyze:
, where r is our interest rate as a decimal, t is time, p is our principal investment and A is our ending value. So, in the context of our problem this becomes:
, which when evaluated by our trusty calculator becomes:
The answer would probably be 18 degrees below normal because the absolute values of the rest are 8 not 18 which is the absolute value of 18 degrees below normal.