31. A black ant and a red ant start from two places 78 meters apart at the same time, moving towards each other. The black ant travels 6 meters per minute, and the red ant travels 7 meters per minute. After 1 minute, they both turn around and move away from each other, then after 3 minutes, they both turn around and move towards each other again, then after 5 minutes, they both turn around and move away from each other again, then after 7 minutes, they both turn around and move towards each other again, …‥ According to this pattern, they will first meet after $\qquad$ minutes.
$48$