Set the distance from the school to the library to x,

Ding Ding starts from home, walking distance 120+x meters, speed 120 meters/minute;

Time =( 120+X)/ 120

Xiaopang starts from school, the walking distance is x meters, and the speed is 100 m/min.

Time =x/ 100

They set out at the same time, arrived at the same time, and owned at the same time.

So: (120+x)/120 = x/100.

X = 600m m.

Time t=600/ 100=6 minutes.