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? Choose the correct choice from the following:	A) 5	B) 6	C) 9	D) 12	E) 15
lamp A flashes every 6 secs,   b flashes every 8 secs,   c flashes every 10 secs.   At certain time three flashes simultaneously.   after that calculate when the three lamps flashes simultaneously in 2 minutes.   take a L.C.M of three 6,8,10 sec then it is 120 secs.   so exactly after 2 minutes it flashes simultaneoulsy.   question stem asks exactly two lamps falshes .   take lcm of a and b .6 and 8 is 24 secs. so two lamp flashes 5times in 2minutes.   lcm of b and c . 8 and 10 secs is 40 secs. so both flashes 3times in 2minutes.   lcm of c and a ,6 and 10 secs is 30 secs . pair flashes 4times..   all three pairs flashes simultaenoulsy at 2 minutes. we have to minus one from all three pairs.   then   12-3= 
<gadget id="calculator">  12-3</gadget>
<output>9</output>
9.   so option 9 is correct
<result>C</result>
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?