Question 1162682
Rate (per hour) of Company B working alone = {{{1/B}}}
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Rate (per hour) of Company A working alone = {{{1/(B+10)}}}
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Rate (per hour) of both companies working together = {{{1/13}}} = {{{1/B}}} + {{{1/(B+10)}}}
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Solve for B:
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{{{1/13}}} = {{{1/B}}} + {{{1/(B+10)}}}
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{{{1/13}}} = {{{1(B+10)/B(B+10)}}} + {{{1(B)/B(B+10)}}}
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{{{1/13}}} = {{{(B+10)/(B^2+10B))}}} + {{{B/(B^2+10B)}}}
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{{{1/13}}} = {{{((B+10)+B)/(B^2+10B)}}}
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{{{1/13}}} = {{{(2B+10)/(B^2+10B)}}}
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{{{1(B^2+10B)}}} = {{{13(2B+10)}}}
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{{{B^2+10B}}} = {{{26B+130}}}
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{{{B^2-16B-130}}} = {{{0}}}
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{{{B}}} = {{{8-sqrt(194)}}} = {{{-5.9}}}
{{{B}}} = {{{8+sqrt(194)}}} = {{{21.9}}}
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You can throw out the negative result, since the rate cannot be a negative number.  Therefore, you are left with the result of 21.9.  
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<b>It takes Company B 21.9 hours to clear the land working alone.</b>