Question 1128070
Ron bought r postcards
Matt bought some other number of postcards, m.

Half of Ron's postcards, or r/2, were given to Matt, who gave 9 to Ron.
Ron now has (r/2)+9.  Matt now has m-9+(r/2)

Each has 21
so Ron has (r/2)+9=21
subtract 9 from each side
(r/2)=12
r=24, multiplying both sides by 2, removing the denominator.

Matt has 21=m-9+(r/2)
21=m-9+12, the 12 from above
21=m+3
m=18 postcards for him

24 and 18
Ron gave 12 to Matt, leaving him with 12, but he got 9 more, giving him 21
Matt had 18, giving Ron 9 leaving him with 9, but he got 12 more, giving him 21.