Solve this question as levels physics waves

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Sound from a small speaker L reaches a point P by two paths which differ in length by 1.2m. When the frequency of the sound is gradually increased, the resultant intensity at P goes through a series of maxima and minima. A maximum occurs when the frequency is 1000 Hz and the next maximum occurs at 1200 Hz. What is the speed of sound in the medium vetween P and L.