With an 8 minute time difference, the clocks will show the same time every 7.5 days (first clock is making up 8 minutes an hour, and needs to catch up 1440 minutes, and 1440/8 = 180 hours or 7.5 days). We also know that the first clock gains 2 hours a day, so we know that when these clocks match up after 7.5 days, they'll show a time of 15:00 (useful to think of a 24 hour clock since we're interested in midnight). Keep adding 15 hours for every 7.5 days to see when they finally match up at midnight (alternatively, find how many times 15 goes into the LCM of 15 and 24 - 120 - and multiply that number by 7.5):
7.5 days - 15:00
15 days - 6:00
22.5 days - 21:00
30 days - 12:00 (noon, not midnight)
37.5 days - 3:00
45 days - 18:00
52.5 days - 9:00
60 days - 24:00 (midnight)
So E.
BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course
Redeem
Featured Course
Target Test Prep GMAT OnDemand
Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.
- 715+ score guarantee — highest in the industry (99th percentile)
- 52 chapters · 1,500+ lessons · 4,000+ practice questions
- 400 hours of video · 1,500+ instructor-led HD smartboard lessons
- 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
- 24/7 live support + weekly Zoom office hours with GMAT instructors
- TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
- 1,200+ flashcards + AI-powered study assistant & daily calendar
- OnDemand, LiveTeach & GMAT Bootcamp formats available
- Also: GRE, SAT Math & Executive Assessment courses
- MBA Admissions Consulting now available
- 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
- 200,000+ students served
- 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★
5.0
(559 reviews)
130-pt guarantee
$0 to start
then $127/mo
Geertrude's Mother
Source: Beat The GMAT — Problem Solving |
Too long. At first I thought I had solved it just by figuring out when the first clock would catch up to the second clock, which took me about a minute. Then I checked the work and realized that wasn't true, and tried to figure out a different way to solve it. When I couldn't do that, I went back to the 7.5 days intermediate solution and used that. Probably 4-5 minutes in all.
-
kevincanspain
- GMAT Instructor
- Posts: 613
- Location: madrid
- Thanked: 171 times
- GMAT Score:790
sourabh33 wrote:
First clock loses 3 min/hour or 1 hour/ 20 hours or 12 hours/ 240 hours
Second close needs only 3/5 of this time to gain 12 hours i.e. 144 hours
LCM (240,144) =720 hours = 30 days
E
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
GMAT Instructor
Gmatclasses
Madrid
You get the same answer because of the "more than three" wording, but it's not actually 30 days. At 30 days, they both hit noon at the same time. You need to do LCM (480,288) instead because you need to gain 24 hours again, not 12.kevincanspain wrote:sourabh33 wrote:
First clock loses 3 min/hour or 1 hour/ 20 hours or 12 hours/ 240 hours
Second close needs only 3/5 of this time to gain 12 hours i.e. 144 hours
LCM (240,144) =720 hours = 30 days
E
-
kevincanspain
- GMAT Instructor
- Posts: 613
- Location: madrid
- Thanked: 171 times
- GMAT Score:790
It says 'strike 12 at midnight', but does not specify whether it has to be 12 o'clock midnight.
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
GMAT Instructor
Gmatclasses
Madrid
