Following figure shows how a dummy activity can be used to represent two concurrent activities, A & B.
By definition, a dummy activity, which normally is depicted by a dashed arrow, consumes no time or resources.
Dummy activity is a hypothetical activity which takes no resource or time to complete.
It is represented by broken arrowed line and is used for either distinguishing activities having common starting and finishing events or to identify and maintain proper precedence relationship between activities that are not connected by events