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At exactly what time past 7:00 will the minute and hour [#permalink]
11 Jul 2003, 18:41

1

This post received KUDOS

6

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00:00

A

B

C

D

E

Difficulty:

75% (hard)

Question Stats:

57% (03:18) correct
43% (03:19) wrong based on 133 sessions

At exactly what time past 7:00 will the minute and hour hands of an accurate working clock be precisely perpendicular to each other for the first time?

(A) 20 13/21 minutes past 7:00 (B) 20 13/17 minutes past 7:00 (C) 21 3/23 minutes past 7:00 (D) 21 9/11 minutes past 7:00 (E) 22 4/9 minutes past 7:00

AkamaiBrah Former Senior Instructor, Manhattan GMAT and VeritasPrep Vice President, Midtown NYC Investment Bank, Structured Finance IT MFE, Haas School of Business, UC Berkeley, Class of 2005 MBA, Anderson School of Management, UCLA, Class of 1993

Re: At exactly what time past 7:00 will the minute and hour [#permalink]
11 Jul 2003, 21:06

Sorry. Guessing not allowed. Show me how you solved it.

(It is NOT that hard if you take the right approach).

Motto: Spoonfed knowledge is not well retained, but knowledge worked hard for will be cherished for a long time.... _________________

Best,

AkamaiBrah Former Senior Instructor, Manhattan GMAT and VeritasPrep Vice President, Midtown NYC Investment Bank, Structured Finance IT MFE, Haas School of Business, UC Berkeley, Class of 2005 MBA, Anderson School of Management, UCLA, Class of 1993

Re: At exactly what time past 7:00 will the minute and hour [#permalink]
11 Jul 2003, 23:04

210 + M/12 - M = 90

Let M be the degrees covered by minute hand from zero. (7:00 pm is the reference point)
initially difference is 210 deg. Hour hand will move at the rate of M/12, so after M minutes hour hand will be at 210+M/12 and minute hand will be at M. The difference should be 90. Find M and divide by 6, which will give you the minutes.

Re: At exactly what time past 7:00 will the minute and hour [#permalink]
12 Jul 2003, 07:49

The answer is D. Evensflow did it the fastest although all of the solutions were fine. The key is computing the rate of each hand and solving for when they are 90 deg apart.

Good job all. _________________

Best,

AkamaiBrah Former Senior Instructor, Manhattan GMAT and VeritasPrep Vice President, Midtown NYC Investment Bank, Structured Finance IT MFE, Haas School of Business, UC Berkeley, Class of 2005 MBA, Anderson School of Management, UCLA, Class of 1993

At exactly what time past 7:00 will the minute and hour [#permalink]
03 Sep 2014, 12:33

3

This post received KUDOS

2

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At 7:00, minute hand must be at 210 degree from 12 (clock wise). Minute hand of the clock moves with 6 degree per minute speed and hour hand moves at 1/2 degree per minute. Therefore in x min hour hand will travel (1/2)*x degree further 210 degree from initial position; meanwhile minute hand must cover 6x degrees. Difference between minute and hour hand is required 90 degree. Formed following equation and solved for x.

Piyush K ----------------------- Our greatest weakness lies in giving up. The most certain way to succeed is to try just one more time. ― Thomas A. Edison Don't forget to press--> Kudos My Articles: 1. WOULD: when to use?| 2. All GMATPrep RCs (New) Tip: Before exam a week earlier don't forget to exhaust all gmatprep problems specially for "sentence correction".

At exactly what time past 7:00 will the minute and hour [#permalink]
12 May 2015, 13:02

Anotehr way to solve this problem is to use the formula that we have for the angle of the clock needels.

\(angle of clock needels =\frac{(60H + 11M)}{2}\) where H is the hour and the M is the minutes. We can imply that the H= 7 because the strating point is 7:00 any way and just need to find the M in the equation with making teh equation equal to 90 since that is what is being asked. Do the math and you will see that \(M=21\frac{9}{11}\)

So not bad to learn the formula at beggiging I was struguling with teh question and trying to go figure out some other stuff, but than I remmeberd on this formula and applied it.

Re: At exactly what time past 7:00 will the minute and hour [#permalink]
20 May 2015, 19:10

PiyushK wrote:

At 7:00, minute hand must be at 210 degree from 12 (clock wise). Minute hand of the clock moves with 6 degree per minute speed and hour hand moves at 1/2 degree per minute. Therefore in x min hour hand will travel (1/2)*x degree further 210 degree from initial position; meanwhile minute hand must cover 6x degrees. Difference between minute and hour hand is required 90 degree. Formed following equation and solved for x.

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