A stationary source of sound A is producing sound of frequency \(170 \mathrm{~Hz}\). Another source of sound B producing sound of frequency \(240 \mathrm{~Hz}\) is moving towards the source A on a straight path with a uniform speed of \(20 \mathrm{~ms}^{-1}\). An observer between A and B is moving towards the source A along the straight path \(\mathrm{BA}\). If the number of beats heard by the observer is zero, the speed of the observer is ____ \(\mathrm{ms}^{-1}\). (Speed of sound in air \(=340 \mathrm{~ms}^{-1}\)).

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.