How to Find Total number of Possible Concurrent Schedules in Transactions?

My friend come up with this idea:- Let T be the total number of transactions, the Total number of possible schedules will be (n + m + k)! /( n! m! k!). While the number of concurrent schedules will be ((n + m + k)!/ (n! m! k!) – T!) .

Question:- Consider 3 transactions T1,T2 and T3 having 2,3 and 4 operations respectively. Find the number of concurrent schedules?

My Answer:- Total no. of concurrent schedules = 9C2 * 7C3 * 4C4 = 1260.

I want to know my friend is correct or me ? should we also count serial schedules in concurrent schedules ? or Something else ? Please clear my doubt !!!!!!!!!!