Prep question

[isisalaska]

Working together at their contant rates, machine A makes 100 copies in 12 minutes and machine B 150 copies in 10 minutes. How many copies would they make in 30 minutes working together?
a) 250
b) 425
c) 675
d) 700
e) 750

[jayhawk2001]

Working together at their contant rates, machine A makes 100 copies in 12 minutes and machine B 150 copies in 10 minutes. How many copies would they make in 20 minutes working together?
a) 250
b) 425
c) 675
d) 700
e) 750

Hmm, I'm not sure I understand the question correctly. Is the first
phrase of the sentence "working together at their constant rates" ?

A makes 100/12 copies in 1 minute
B makes 150/10 copies in 1 minute

A and B together can make 100/12 + 150/10 copies in 1 min

So, in 20 minutes, they can make 20 * (100/12 + 150/10) = 466.67
copies.

This is obviously not one of the answers. I'm stumped.

[isisalaska]

My fault, it should read 30 minutes instead of 20. I changed it.

So the answer is 700
100/12 + 150/10= 8.5+15= 23.33 x 30min. = 700 copies

[Neo2000]

If you have observed the numbers carefully

MAchine A will make 50copies in 6minutes or 50x5 copies in 6x5 mins=
250copies in 30mins

Similary Machine B will make 150x3 copies in 10x3 minutes

So total number of copies = 250 +450 = 700

[Cybermusings]

Working together at their contant rates, machine A makes 100 copies in 12 minutes and machine B 150 copies in 10 minutes. How many copies would they make in 30 minutes working together?
a) 250
b) 425
c) 675
d) 700
e) 750

A --- 100 copies in 12 min
or 100/12 copies in 1 min

B ---- 150 copies in 10 min
or 15 copies in 1 min

30 min --- A 100*30/12 copies and B will make 15*30 copies
= 250 + 450 copies
= 700 copies