Suppose an average traveler's tastes can be described by the utility function u(x1, x2, x3)=x1x3+x2, where x1 is miles travelled by air, x2 is other consumption, and x3 is an index of air safety that ranges from 0 to 100. Calculate the MRS of other goods for airline miles; i.e., the MRS that represents the slope of the indifference curves when x1 is on the horizontal and x2 is on the vertical axis.