SAT数学题,请问这题什么意思The table above shows how many students in a

SAT数学题,请问这题什么意思
The table above shows how many students in a class of 12 preschoolers had 0,1,2, or 3 siblings. Later, a new student joined the class, and the average (arithmetic mean) number of siblings per student became equal to the median number of siblings per student. How many siblings did the new student have?
图就不上了,请给我翻译一下