Question 1171222
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one machine can complease a job in 10 minutes if the same job is done by this machine and an older machine working together the two job can be completed in 6 minutes. 
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Maybe this is written better:

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One machine can complete a job in 10 minutes. If the same job is done by this machine and an older machine working together the {{{cross(two)}}} job can be completed in 6 minutes. 
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If this is really the way you mean the problem then:
{{{1/10+1/t=1/6}}}

{{{60t(1/10+1/t)=(60t)(1/6)}}}

{{{6t+60=10t}}}

{{{4t=60}}}

{{{highlight(t=15)}}}-------------minutes if just the older machine does 1 job alone.