回复：回复：15 is # of combination of 4 numbers out of 6, 20 is 3 out

wow! thanks for the RPECISE explanation after coming back from lunch! you're my honored student!!! -- just kidding :)

q2 is similar idea,琼岛春荫 if you son/daughter grasps the basics explained above, he/she will be able to figure out on his/her own. I'll share my answer once i get to it.

p.s. 20 is to pick 3 out of 6, as we know 1st position is taken.