3x+15 = 10 is a simple equation. If 5 is added to the left-hand side of the equation, the equation remains the same as the given equation.
If 5 is added to the left-hand side of the equation 3x+15 = 10, we get 3x+15+5 = 10. The value on the left-hand side is increased but it remains same on the right-hand side. So, equality doesn't hold good now. 3x+15+5 ≠ 10. Equality sign remains unchanged only when same quantity is added on both sides i.e., 3x+15+5 = 10+5. Thus, the equation does not remain same as the given equation.