96 is the correct answer but I don't know hot to get to that answer

1. Substance A (red) changes to substance B (blue) in a first-order gaseous reaction. The scenes below represent the reaction mixture at three different times: 1: 16 red spheres at time of 0 minutes. 2:8 red, 8 blue spheres at time of 24 minutes. 3: 1 red, 15 blue spheres at time unknown. What is the time at which the third scene occurs? 96 mins 72 mins t2453{3} 48 mins 144 mins 192 mins