TeymurHajiyev wrote:

Lamp A flashes every 6 seconds, Lamp B flashes every 8 seconds, Lamp C flashes every 10 seconds. At a certain instant of time all three lamps flash simultaneously. During the period of 2 minutes after that how many times will exactly two lamps flash?

A) 5

B) 6

C) 9

D) 12

E) 15

I've found different answer than an official one. What do you think, guys? What would be your answer?

Hi,

since 120 secs is too small a time, one can work out each time two lights are on...

lets see the way/procedure incase the time frame is much more..

we will solve this Q by that procedure..

we have three different time at which the bulbs are on.. 6,8,10..

the solution is based on LCM concept..1) we will get the entire time in same units, that is minutes..

2) take LCM of all three, this along with its multiples till the total time, will be the time all three will be on simulatenously..

to find how many times all three are on simultaneously, divide the TOTAL TIME by LCM, the INTEGER value will give us the number, say A..

3) Next would be to take LCM of each pair of time and divide total time by each of these three LCMs. Again the integer value will give us the time two blink together..

4) Since we are asked exactly 2 of them blink together, we subtract A, all three blinking together, from each of the three answers we get from (3) above.

5) Add all three new values and we have the answer....

lets use this in this Q..1) total time=120 secs..

2) LCM of 6,8,10=120.. times=120/120=1... so all three will switch on together only once , A=1..

3) three pairs..

6,8.. LCM=24...times=120/24=5

6,10.. LCM=30..times=120/30=4

8,10.. LCM=40..times=120/40=3

4) three sets of two blinking together

5) total times= 4+3+2=9

_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372

Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

GMAT online Tutor