A law for the velocity of chemical reactions states that the amount of chemical change per unit of time is proportional to the mass of changing substances present in the system. The rate at which the change takes place is proportional to the mass of the substance still unchanged. If denotes the original mass, find an expression for the mass remaining unchanged after a time has elapsed.
Source: March, Herman W., and Wolff, Henry C. (1917). Calculus. New York: McGraw-Hill. (Question 1, p. 158)