he is working on math problems

mathematica\u8f6f\u4ef6\u2014Wolfram\u6388\u6743\u6b63\u7248\u8f6f\u4ef6\uff0c\u4ef7\u683c\u4f18\u60e0 1 on work on做（work out算出的话就不能用ing了）
2 How many 多少,classes可数名词复数
3 working on problems
4 C 现在进行时
5 else 其他的

扩展阅读：he has been working ... he is very help at home ... he is hard working ... economic ... he working very ... why did he stop working ... he still kept on working ... he is strict too ... i wanna hold you ...