Four harmonic waves of equal frequencies and equal intensities I _...

Let amplitude of individual wave is A
Then by using phasor method


So resultant amplitude ${A}_{r}^{2} = {A}^{2} + {A}^{2} + 2{a}^{2}\cos 60$
$∴ {A}_{r}^{2} = 3{A}^{2}$
$∴ {I}_{r} = 3{I}_{0}$