Please see the solution video, which shows that the wave pulses interfere when they are in the same position, but then continue afterwards, looking and moving the same as they would have without the previous interference.
This is College Physics Answers with Shaun Dychko. The period of oscillation of these swinging parakeets will be two pi times the square root of the distance from the pivot to their center of mass, divided by acceleration due to gravity. Now, frequency is the reciprocal of period so if we take the reciprocal of this which is the same as raising to the exponent negative one, or flipping each factor. We get one over two pi times square root <i>g</i> over <i>L</i>. So it's one over two pi times square root 9.81 meters per second squared, divided by ten centimeters written as ten times ten to the minus two meters. This gives 1.58 Hertz.