Printing press A can print an order in 7hours. If printing press B is used
along with press A, the job can be completed in 5hr. How long would it take
printing press B to print the job by itself?
Make the chart below. It is just like a D=RT chart except that
D stands for "DEEDS" not "DISTANCE". In other words, D stands
for the number of DEEDS or printing jobs done in each case.
No of jobs Rate in jobs per hour Time in hours
Press A only
Press B
Both presses
------------------------
We read:
>>...press A can print an order in 7 hours...<<
So that's 1 job in 7 hours. So we fill in 1 for the number
of jobs which A alone does and 7 for the number of hours.
No of jobs Rate in jobs per hour Time in hours
Press A only 1 7
Press B
Both presses
-------------
Next we read:
>>...If printing press B is used along with press A, the job can be
completed in 5hr
That's 1 job in 5 hours for both presses. So we fill in 1 for the number
of jobs which both presses together do and 5 for the number of hours.
No of jobs Rate in jobs per hour Time in hours
Press A only 1 7
Press B
Both presses 1 5
-----------------------
Now we read the question:
>>...How long would it take printing press B to print the job by itself?...<<
That asks how many hour would it that press B to do 1 job. So we
let x be the number of hours press B would take to do 1 job. Se
we efill in 1 nfor the number of jobs and x for the number of hours.
No of jobs Rate in jobs per hour Time in hours
Press A only 1 7
Press B 1 x
Both presses 1 5
----------------------
Now we fill in the rates by using the equivalent of
DISTANCE
RATE = ----------
TIME
which is:
NUMBER of JOBS
RATE = -----------------
TIME
No of jobs Rate in jobs per hour Time in hours
Press A only 1 7
Press B 1 x
Both presses 1 5
Now we use the formula:
Rate of Press A + Rate of Press B = Rate of Both presses together
+ =
Can you solve that? You have to multiply through by the
LCD of 35x. If you can't solve it post again asking how.
Answer: 17 hours.
Edwin