The system of equations that is solved by substitution is done by writing one equation in terms of one variable and substituting this expression to the other equation. It is done as follows:
2x+3y=105
x = 105/2 - 3y/2
x+2y=65
105/2 - 3y/2 +2y = 65
1/2y= 25/2
y = 25