13+5= 18. After the boys robbed it, there were 18 little sisters, leaving 28- 18= 10.

10x2=20。 After my sister robbed me, there were 20, that is, the boys left 28-20=8.

8x2= 16, that is, 16 root that the young man started to divide.

That is, the little sister started 28- 16= 12.

If you learn the equation, it will be easier.