Natalee's mistake is that she has written the function incorrectly. In her function, she has multiplied the starting number of bacteria, 100, by the growth rate, 40%, to get the final number of bacteria. However, this is not correct because a growth rate of 40% means that the number of bacteria increases by 40% of the starting number, not by 40 itself.
To write the correct function, we can use the fact that a growth rate of 40% means that the final number of bacteria is 1.4 times the starting number. Therefore, the correct function for this scenario is y = 1.4 * 100 = <<1.4*100=140>>140. This function shows that the number of bacteria increases to 140 after experiencing a 40% increase in growth.