If A+B< 10 , the solutions is A+B=C (1,2,3) (1,3,4), so on..............
If A+B>or = 10 , AAA+BBB would be 4 digits, so it wont work
Question 1: AAA
+BBB
------
CCC
Question 2: MMM
NNN
+ PPP
------
QQQ
Question 3: TTT
x S
------
RRR
Question 4: ABC
+DEF
------
GHI