Question 416346
Add their rates of copying
In words:
( 1 copy / 8 min ) + (1 copy / 10 min) = (1 copy / x min )
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{{{ 1/8 + 1/10 = 1/x }}}
Multiply both sides by the LCD, {{{40x}}}
{{{5x + 4x = 40}}}
{{{9x = 40}}}
{{{x = 40/9}}}
{{{x = 4.444}}} min 
or about 4 min 27 sec
check answer:
{{{1/8 + 1/10  = 1/(40/9)}}}
{{{.125 + .1 = 9/40}}}
{{{.225 = .225}}}
OK