oh man.. silly mistake...
thanks for correction.
4C1 = number of ways one senior can be selcted
9C2 = number of ways any 2 people can be selected from remaining 9 people.
Thus ways to make groups of 3 people groups have atleast 1 senior.
hope this clarifies.
1. C ...both are required to solve the problem
2. 100 is the answer..
Most simplest way is 10C3 - 6C3
Vicks...did you notice the ATLEAST one senior part in the problem
Question, why do you multiply the combinations?