Explanation: the multiplicative rate is the ratio of the values y(k)/y(k+1) (with k being the index). This ratio must be the same for the function to be exponential. We can take y(2)/y(1) = 4/6 = 2/3.
100% is equal to $16 so 5% would be 16/20 or $0.80 then to get 15% you would multiply $0.80 by 3 which is $2.40. This is the tip. Then do $16 + $2.40 and the total + tip is $18.40. (Double check to make sure I'm correct)
