Explanation:
so, the function is
C(t) = 24(0.87)^t °
the initial temperature is after 0 minutes (that means t = 0).
C(0) = 24(0.87)⁰ = 24×1 = 24°
the sandwich had an initial temperature of 24° C
since t is the exponent of a number smaller than 1 (0.87), the bigger t gets the smaller the result becomes.
so, it is a decay. as expected, as the sandwich in the cooler should get cooler and cooler over time.
the % of change is always in the multiplication factor.
e.g. if the factor is 1.06 of a value x, then the value increased by 6% (the 100-fold difference to 1).
the same way, if the factor is 0.93, then the value decreased by 7% (again, the 100- fold difference to 1).
in our case, the difference from one minute to the next is a multiplication by 0.87, as
0.87^t × 0.87 = 0.87^(t+1)
therefore, the percent the temperature changes is 13%.