Question 599396
C(n,r) = (n!)/(r!(n-r)!)


C(15,12) = (15!)/(12!*(15-12)!)


C(15,12) = (15!)/(12!*3!)


C(15,12) = (15*14*13*12!)/(12!*3!)


C(15,12) = (15*14*13)/(3!)


C(15,12) = (15*14*13)/(3*2*1)


C(15,12) = (2730)/(6)


C(15,12) = 455


So there are <font color="red">455</font> ways.