So i am doing stuff with mutually exclusive and inclusive probabilities. For some reason, whatever i do, i seem to do it wrong. For example, this problem says "There are 5 pennies, 7 nickels, and 9 dimes in an antique coin collection. If two coins are selected at random and the coins are not replaced, find P(2 nickels or 2 silver colored coins)." So...what i thought was to find the two probablities (7 nCr 1 * 6 nCr 1/ 21 nCr 2)+ (16 nCr 1 * 15 nCr 1/ 21 nCr 2), which gets me something like 937/1470, while the answer is 4/7. What am i doing wrong? And could anyone show it in this problem possibly? "There are 5 male and 5 female students in the executive council of the Douglas High School honor society. A committee of 4 members is to be selected at random to attend a conference. Find P(at least 3 females)." Thanks a bunch!