number of ways to select 7 coupons = C(15,7).
we select 9.
the rest 6 to be selected from 1 - 8.
ways = C(8,6).
probabilty is C(8,6) / C(15,7) ???
galat ho sakta hai. probability nahi kari acche se..
9^7/15^7 is the anser
ory puka nuvuu keka
i didn't read the word - "replacement"!!!
i had ot read the word with replacements.
so now i know what it means,
number of ways to pick up 7 coupons is 15^7. cause we have 15 choices on each time we pick.
there 9^7 ways to pick the coupons numbered 1 to 9.
we want at least one 9.
therefore number of ways = (total ways) - (ways where we don't have 9 and select from 1 to 8) = 9^7 - 8^7.