Question 990397
<pre>
printers       copies   time    rate
printer         1        2.5    1/2.5
printer         1        1.5    1/1.5
both at once    1        x      1/x
</pre>


All rates are using 1 copy but different time amounts.  The x is for time in minutes for 1 copy; and the individual rates are additive to make the combined rate.


{{{1/2.5+1/1.5=1/x}}}


{{{1/(5/2)+1/(3/2)=1/x}}}

{{{2/5+2/3=1/x}}}
Simplest common denominator,  15x.
{{{(2/5+2/3)(15x)=15}}}
{{{6x+10x=15}}}
{{{16x=15}}}
{{{highlight(x=15/16)}}}---------THIS is how many minutes for the two printers together to make 1 copy; *but in practice, nobody uses more than one printer to make just one copy.  This is really 16 copies per each 15 minutes.