So lets say task B is dependent on Task A, hence Task A has sequence value 10 and Task B has sequence value 20 (Task A preceeds TaskB). Task A branch type should be conditional and for the condition formula should be written in to the "condition or probability" fieldç
For Task B enter the processing time and other data as usual. If the auto-cancel trigger is set to "All immediate predecessors cancelled" (as it is by default), Task B will be cancelled if the condition for Task A didnt return true value.
Hope this helps