15. A member of a collegiate track-and-field team recently adopted a new training regimen. His times, s, in the 400-meter event began to improve as described by the function S= 2.40 1+ekt +46, where s is his time and t is the number of months since beginning the new training regimen. Determine k if his times have improved 1.0 second after 6 months. (Substitute t=0 to determine his 400 m time before the training change.) Round to the nearest thousandth.