math question??!!!! need help???

Question

Two Machines, one job:
the old machine could shred a truckload of paper in 4hrs.

the new machine could shred the same truckload of paper in only 2hrs.

How long will it take to shred a truckload of paper if ron runs both shredders at the same time?

dont just tell me with logic..i want the math formula and stuff...i know that the formula should be set up like
x/2+x/4=1
i just dunno how to go from there...

Answer 1

You need to use the product over sum method:

ole machine 4 hours = A
new machine 2 hours: + B

A*B
------
A + B

on plug in numbers:

2*4
--------
2 + 4

8
------
6
now divide 8 by six:

and answer is:

1 with 2 left over
so
1 and 2/6 hours now reduce fraction

1 1/3 hours with both machines working in conjunction with
(1.333333----hours in decimal form. )
each other.

Answer 2

It should take 2 hours. Becuase he's running them at the same time, so he's gonna have to put half of the papers in the old one, and half in the new one. So if it takes 4 hours in the old one, half of that is 2 hours. While at the same time in the new machine it should take half the original time which is 1 hour. So while the old shredder is running, the new one will finish 1 hour before the old one that takes 2 hours. i hoped i helped, and sorry if i complecated things.

Answer 3

1 hour for the first and 2 hours for the second