Question 165668
Since the laser printer can print 1 page in 15s, it follows that it can print 16 pages in 240s. With this 16-page job taking both printers 50s, it means, obviously that the older printer is faster than the laser printer.

Since the laser can print 16 pages in 240s, then it can print {{{(1)/(240)}}} of the job in 1 sec.

To find what fraction of the 16-page job the old printer can do in 1 sec., when doing the job with the laser, we have: 
{{{((1)/(240)+(a)/(1))/(1)=(1)/(50)= (5)/(24)+(50a)/(1)=1}}}

5  +  1200a  =  24
1200a   =   19
a  =  {{{(19)/(1200)}}}

Since in 1 sec., the older printer does {{{(19)/(1200)}}} of the job, then it can do the entire job in:
{{{((19)/(1200))/(1)=(1)/(a)=(19a)/(1200)=1}}}

19a  =  1,200

a  =  {{{(1200)/(19)}}}  =  63.15789

This means that by itself, the older printer can do the entire 16-page job in 63.15789 seconds, which means that it can do 1 page in {{{(63.15789)/(16)}}}  =  3.947368  ≈  3.947 seconds.