Mr. Rr the Rreliable Rrobot has been programmed to whistle every 18 seconds and do a jumping jack every 42 seconds, starting from the moment he is turned on. (For example, he does his first jumping jack 42 seconds after he is turned on.)

How many times during the first 15 minutes after activation will Mr. Rr whistle and do a jumping jack at the same instant?

Question

Mr. Rr the Rreliable Rrobot has been programmed to whistle every 18 seconds and do a jumping jack every 42 seconds, starting from the moment he is turned on. (For example, he does his first jumping jack 42 seconds after he is turned on.) 

How many times during the first 15 minutes after activation will Mr. Rr whistle and do a jumping jack at the same instant?

anonymous · Answer

find the common multiples of 15 and 42 that are below 15x60=900(seconds)

amilapsn · Answer

126
252
378
504
630
756
882

amilapsn · Answer

7 times?