The correct way to simplify the expression is...
1) Subtract 5c from both sides of the equation
5c+16.5=13.5+10c
-5c = -5c
16.5=13.5+5c
2) Subtract 13.5 from both sides of the equation
16.5= 13.5+5c
-13.5=-13.5+5c
13 = 5c
3) Divide 13 and 5 by 5
13=5c
13/5=2.6 and 5/5=1
Therefore 2.6=c
Hope this helped!