We know that The formula for combinations is C=n!/[(n-r)!*r!] where n is the total number of objects you choose from r is the number that you choose to arrange
in this problem n=15 students r=4 students C=15!/[(15-4)!*4!]-----> C=15!/[11!*4!]---> (15*14*13*12*11!)/(11!*4*3*2*1) C=(15*14*13*12)/(24)----->C=1365
