BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

rates

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 245
Joined: Thu Aug 16, 2007 12:41 pm

rates

by bacali » Mon Dec 01, 2008 3:29 pm
Machine A, operating alone at its constant rate, produces 500 feet of a particular fiber in 2 hours. Machine B, operating alone at its constant rate ,produces 500 feet of the same fiber in 3 hours. Machine C, operating alone at its constant rate. Produces 500 feet of the same fiber in 5 hours. How many hours will it take machines A. B, and C, operating together at their respective constant rates, to produce 1000 feet of the fiber?



A. 60/31 B. 15/7 C. 25/11 D. 10/2 E. 20/3



OA: A
Top
Source: — Problem Solving |
User avatar
Community Manager
Posts: 1049
Joined: Sun Apr 06, 2008 5:15 pm
Location: Pittsburgh, PA
Thanked: 113 times
Followed by:27 members
GMAT Score:710

by dmateer25 » Mon Dec 01, 2008 3:52 pm
Take the inverse of each of these rates and set them equal to (1/t) to find the time it takes working together.

For 500 feet:

1/2 + 1/3 + 1/5 = (1/t)

15/30 + 10/30 + 6/30 = 1/t

31/30 = 1/t
30/31=t

so for 500 ft it takes 30/31. For 1000 ft it would be (30/31)*2=60/31
Top
Legendary Member
Posts: 2467
Joined: Thu Aug 28, 2008 6:14 pm
Thanked: 331 times
Followed by:11 members

by cramya » Mon Dec 01, 2008 8:06 pm
Another way

Rate A = 500/2= 250 (FOR 1 HOUR)
Rate B = 500/3 (FOR 1 HOUR)
Rate C = 500/5 = 100

Add the rates when they work together

rate*time = work
(250+500/3+100)*time = work
(250+500/3+100)*time = 1000

time = 3000/1550 = 60/31

A)
Top
Post new topic Post Reply
• Page 1 of 1