How can I actually delegate a complex task without it turning into me just doing it myself?

Keeping it high level, my boss asked me and my staff to perform a complex reconciliation between two large excel files, and she asked us to work together to get it done faster.

As we were going through, I told her the process, walked through some examples with her, and then told her which sections to work on while I worked on my portions. When we regrouped, she had done everything incorrectly. Part of being a good manager is recognizing that some tasks are a stretch for some people, so I wasn't mad, but I again showed her what she needed to do, and eventually we got there.

But, in order for me to have assurance that everything was correct, I pretty much had to do her portions on my own and then compare her results to what I did. I feel like the task would have gone faster, and I would have had greater assurance in the final product, if i just did it all myself and if there was just one brain keeping everything in one working memory.

Has anyone else experienced something like this? How do you delegate something / work together on something complex without ending up doing it all yourself?

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