The correct answers to L and Vb are 9020m and 820m/min.
The initial guess values are 10m and 10m/min, these guess value doesn't give good answers.


I add precision condition in FindRoot, but it doesn't help.


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Wrote

The initial guess values are 10m and 10m/min, these guess value doesn't give good answers.

Giving guess values for FindRoot() and roots() are more restrictive.
Whatever you do, you may need to change your guess values to fit.
Sometimes, some other plugins may work as intended though.
It is better to cross-check your results.
And plotting is essential imho.

Regards