Solution:
As per the question,
18 = No. of mice
3 = No. of groups
As the groups are equally large,
No. of mice in each group 
mice
No. of ways of placement of mice 
The placement of mice for each group 
As a result, the number of ways required 
18 mice were placed in two experimental groups and one control group, with all groups equally large. In how many ways can the mice be placed into three groups?
18 mice were placed in two experimental groups and one control group, with all groups equally large. In how many ways can the mice be placed into three groups?