D. is NOT correct. Hint: Say you buy stock for X amount. It then falls by 25%, then subsequently rises by 25%, do you get back to the same place?
_________________

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

The correct answer is (A). Clock #1 is 15 minutes slow. This means that after one actual hour, the clock shows that only 45 minutes have gone by.

Clock #2 is 15 minutes fast relative to Clock #1. That means that after one hour on Clock #1, Clock #2 moves ahead 60 + 15 or 75 minutes. This ALSO means that Clock #2 is running 75/60 = 5/4 times as fast as Clock #1. Therefore, after one actual hour of elapsed time, Clock #1 moves 45 minutes and Clock #2 moves 45 * 5/4 minutes.

Clock #3 is 20 minutes slow relative to clock one. That means that after one hour on Clock #2, Clock #3 moves ahead 60 - 20 or 40 minutes. This ALSO means that Clock #3 s running 40/60 = 2/3 times as fast as Clock #2. Hence, after one actual hour of elapsed time, Clock #1 moves 45 minutes, Clock #2 moves 45 * 5/4 minutes, and Clock #3 moves 45 * 5/4 * 2/3 minutes.

Clock #4 is 20 minutes fast relative to Clock #3. That means that after one hour on Clock #3, Clock #4 moves ahead 60 + 20 or 80 minutes. This ALSO means that Clock #4 is running 80/60 = 4/3 times as fast as Clock #3. Hence, after one actual hour of elapsed time, Clock #1 moves 45 minutes and Clock #2 moves 45 * 5/4 minutes, Clock #3 moves 45 * 5/4 * 2/3 minutes, and Clock #4 moves 45 * 5/4 * 2/3 * 4/3 = 50 minutes which is equivalent to saying that Clock #4 loses 10 minutes per actual hour.

At 6 p.m., six actual hours have gone by since all of the clocks were reset, hence Clock #4 loses 6 * 10 = 60 minutes, i.e., Clock #4 is an hour slow. Hence, the apparent time on Clock #4 is 5:00 p.m. and the correct answer is (A).
_________________

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

A not-so-good clockmaker has four clocks on display in the window. Clock #1 loses 15 minutes every hour. Clock #2 gains 15 minutes every hour relative to Clock #1 (i.e., as Clock #1 moves from 12:00 to 1:00, Clock #2 moves from 12:00 to 1:15). Clock #3 loses 20 minutes every hour relative to Clock #2. Finally, Clock #4 gains 20 minutes every hour relative to Clock #3. If the clockmaker resets all four clocks to the correct time at 12 noon, what time will Clock #4 display after 6 actual hours (when it is actually 6:00 pm that same day)?

(A) 5:00
(B) 5:34
(C) 5:42
(D) 6:00
(E) 6:24


could you explain your method please?

Even I found the answer to be A. Is this a GMAT problem ? I took more than five mins to solve this rather understand this.

6 * ( 1-1/4) (1+1/4)(1-1/3)(1+1/3)
1/4 hr is 15 minutes and 1/3 hr is 20 minutes

6 * ( 1-1/16) * ( 1-1/9 ) = 6 * 15/16 * 8/9 = 6 * 5/6 = 5
Sothe 4th clock should just display 5 hrs after 12