Question 1128070
Let R = Ron's postcards before the trade
Let M = Matt's postcards before the trade
After the trade, Ron has R/2 + 9 since he traded half and received 9 from Matt
After the trade, Matt has M - 9 + R/2 since he gave away 9 and received half of Ron's
Both have 21 after the trade.  In equation form this is:
R/2 + 9 = 21
M - 9 + R/2 = 21
The first equation gives R = 24
Solving for M in the second equation we get:
M - 9 + 12 = 21
This gives M = 18
Ron has 24, Matt has 18