.
In order to be correct,
the condition MUST SAY "how long it would take them TO DO WHAT ? if they worked together".
So take the labor to formulate the condition correctly.
Please. And thank you.
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Looking on the @josgarithmetic efforts, I am wondering every time,
how this person, who can not practically solve correctly any problem,
is ready to solve enthusiastically even incorrectly formulated problems ! !