>>11096168holy shit this entire thread is proof that sci is retarded
literally consider two sides of (1+x)^m(1+x)^n=(1+x)^{m+n}
alternatively do the brainlet thing: literally read the equality out loud and you ll know that it's true:
"on the right, i choose k people out of a group of n girls and m boys. on the left, i can either choose 0 girls, 1 girl, 2 girls,..."
