题目

Tom was a seven— year— old boy.He was sick to death of school because he didn't want to do his homework.He was always looking for excuses not to do it.When he sneezed or had a headache, he told his mom he was seriously ill and he couldn't do his homework.Though Tom's mom was angry, she didn't know how to deal with him.Most of the time, Tom went to school without finishing his homework.Even though he finished his homework, he usually did it very carelessly.One Thursday evening, Tom told his mom that he couldn't do his homework because he was having a fever.After taking his temperature, Mom 六年（1）班分三个小组参加植树活动，甲组植树60棵，是乙组植树棵树的45，丙组比乙组少植215．丙组植树多少棵？