Fundamentals: Principle of superposition

When two waves meet , you sum the displacement of the two waves. The resultant wave will be 4 times the intensity of individual wave as amplitude is 2A for constructive interference ( when the wave are in phase). For destructive interference (wave are antiphase) the intensity is zero as the resultant amplitude is zero. This is the ideal case only. In reality, intensity reduces with distance and amplitudes are not equal . So for observable interference the amplitudes must be approximately equal.