Write the wave pair in standard form Start from y1 = a1 sin wt and y2 = a2 sin (wt + phi) so you can directly map amplitude and phase data from the question.

Lock the coherence condition Check that sources have same frequency with stable phase difference; if coherence is absent, persistent interference logic cannot be applied.

Use phase and path conditions separately For maxima use phi = 2npi and path difference nlambda, and for minima use phi = (2n-1)pi with odd half-wavelength path difference.

Compute amplitude before intensity ratio Find A from A = sqrt(a1^2 + a2^2 + 2a1a2 cos phi) first, then translate to intensity using I proportional to A^2 to avoid ratio errors.

Interpret energy statement correctly Mark that energy is redistributed, not destroyed; use this to reject distractors claiming net energy loss in destructive regions.