Question 110413
First I have to assume that Tina and Gina are the same person, otherwise this problem has no unique solution, and I'm going to call her Tina.
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Tina working alone could spray the orchard in 12 hours, so that means that Tina can spray {{{1/12}}} orchards per hour.
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Tina and Cal, therefore, spray {{{1/8}}}orchards per hour together.
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And finally, Cal can spray {{{1/x}}} orchards per hour by himself.
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{{{1/12+1/x=1/8}}}
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The lowest common denominator for these three fractions is 24x, so:
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{{{2x/24x+24/24x=3x/24x}}}
{{{2x-3x=-24}}}
{{{x=24}}}
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So Cal would take 24 hours to do the job by himself.