Work or task rates.
One job is "create the 30 lessons".
Terry 1/(x-60)
Charles 1/x
Both at same time 1/300
The alternative way is to take the equivalent, Charles needs 60 hours MORE than Terry, and form the rates this way:
Terry 1/x
Charles 1/(x+60)
Both at same time 1/300
Both together, their rates are......
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, but the MINUS form will not make sense.