If Zuhr is missed and the time remaining for Asr is too short to perform both prayers, the current Asr is offered first and the missed Zuhr afterwards.
With ample time and applicable conditions, the earlier missed prayer ordinarily has precedence.
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Worked example: protecting the current prayer
Making Up Missed Prayers: Obligation and Sequence
If Zuhr is missed and the time remaining for Asr is too short to perform both prayers, the current Asr is offered first and the missed Zuhr afterwards.
With ample time and applicable conditions, the earlier missed prayer ordinarily has precedence.