Let Lele jump rope x times per minute, then Jiajia jumps rope (x- 12) times per minute.

The equations are listed according to the meaning of the problem.

2x+3(x+x- 12)=780

Then x= 102

So Lele jumps 102 ropes per minute, and Jiajia jumps 90 ropes per minute, so Lele jumps 240 more than Jiajia.