Question 157874
The first reader can do 1/4 of the job in an hour. (0.25 job/hour)
The two together can do 1/2.5 of the job in a hour. (0.4 job/hour)

You are asked to find the work rate of reader 2.

{{{readerOneRate + readerTwoRate = bothRate}}}
{{{0.25 + r2 = 0.4 }}}
{{{r2 = 0.4 - 0.25}}}
{{{r2 = 0.15}}}

So reader 2 can do 0.15 job/hour.
How many hours to do the whole job?
h*0.15 = 1
h = 1/0.15
h = 6.666 hours (or 6 hours and 40 minutes)