PLEASE HELP
i need to give the reason and statement for
GIVEN:RS= 1/2RT
PROVE: S is the midpoint of line RT
so far i have this.. i need help with number 4 and need someone to go over it and see if everything else is correct please..
STATEMENT REASONING
1) RS= 1/2RT given
2) 2RS=RT multiplication property
3) RS+ST=RT Segment Addition postulate
4) 2RS=RS+ST ?
5) RS+ST Division Property
6) S is the midpoint Defintion of midpoint

Question

PLEASE HELP 
i need to give the reason and statement for 
GIVEN:RS= 1/2RT
PROVE: S is the midpoint of line RT
so far i have this.. i need help with number 4 and need someone to go over it and see if everything else is correct please..
     STATEMENT               REASONING
1)   RS= 1/2RT                    given
2)    2RS=RT             multiplication property
3)   RS+ST=RT      Segment Addition postulate
4) 2RS=RS+ST                        ?
5)  RS+ST                        Division Property
6) S is the midpoint     Defintion of midpoint