For anyone who can't remember why, for example,
C(11,3)*C(39,2) should equal 122,265 this might help:
C(n,r) is often written as nCr
and it equals (n!)/[(n-r)!r!]
(note: n! is spoken as "n factorial" and means multiplying all the numbers from 1 up to n together; so if n = 4, n factorial would mean 1*2*3*4 or 1x2x3x4 or 4x3x2x1 or 24)
which for 11C3 means (11x10x9x8x7x6x5x4x3x2x1)/[(11-3)!3!]
or (11x10x9x8x7x6x5x4x3x2x1)/(8x7x6x5x4x3x2x1)x(3x2x1)
(note: you can simply cross off the (8x7x6x5x4x3x2x1) from the end of the (11x10x9x8x7x6x5x4x3x2x1) without having to multiply them both out, which will give
or (11x10x9)/(3x2x1)
or 165
and for 39C2 means (39x38x...)/[(39-2)!2!]
or (39x38x...)/[(37x36x...)x(2x1)]
or (39x38)/(2x1)
or 741
so 11C3*39C2
or C(11,3)*C(39,2)
or C(11,3) x C(39,2)
or 165 x 741
equals 122,265.
I hope is useful to someone, and if you save these calculations they might make the next combinations thread a little easier to understand.