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Bunuel
Marie has two electronic beepers: one beeps 5 times in every 90 seconds and the other one beeps 6 times in every 96 seconds. If both the beepers are started simultaneously, how many times will they beep together in next 3 hours 35 mins and 16 seconds?

A. 86
B. 87
C. 88
D. 89
E. 90

Using the LCM, we can derive when both beepers will ring for the first time,i.e 144 seconds.
So, 3 hours 35 mins and 16 seconds will translate into 12916 seconds.

Thus, 12916/144= 89.5 seconds.
Hence IMO D
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Bunuel
Marie has two electronic beepers: one beeps 5 times in every 90 seconds and the other one beeps 6 times in every 96 seconds. If both the beepers are started simultaneously, how many times will they beep together in next 3 hours 35 mins and 16 seconds?

A. 86
B. 87
C. 88
D. 89
E. 90
Solution:

We see that beeper 1 beeps every 90/5 = 18 seconds and beeper 2 beeps every 96/6 = 16 seconds. Since the LCM Of 18 and 16 is 2 x 9 x 8 = 144, the 2 beepers beep simultaneously in 144 seconds. Since 3 hours 35 mins and 16 seconds = 3 x 3600 + 35 x 60 + 16 = 12,916 seconds, the two beepers beeps [12,916/144] = [89.7] = 89 times together. (Note: The notation [y] means the greatest integer less than or equal to y.)

Answer: D
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