Question

please answer Q.6 and Q.7

Answer 1

Q7. They will beep again in LCM(15, 20) minutes.

So, 15=3*5

and, 20=2*2*5

LCM(15,20)= 5*6*2 = 60

So, They will beep after 60 minutes. They beeped at 6a.m. now they will beep at 7a.m.

Q6. Given that traffic light at three different road crossing change after every 48 seconds,72 seconds and 108 seconds respectively. 
48 = 2 × 2 × 2 × 2 × 3
72 = 2 × 2 × 2 × 3 × 3 
108 = 2 × 2 × 3 × 3 × 3 Hence LCM of 48, 72 and 108 is (2 × 2 × 2 × 2 × 3 × 3 × 3)  = 432.
That is after 432 seconds they will change simultaneously.
432 seconds = 7 min 12 seconds. 
Thus the traffic lights change simultaneously at 7:7:12 am.

Hope this helps...

Answer 2

LCM of 15,20=60
so,after 60 min the bip together so answer is 7a.m.