Find the rate in which both balls make one full rotation around the track.
Ball A:

It takes 30 seconds for Ball A to make a full rotation.
Ball B:

It takes 20 seconds for Ball B to make a full rotation.
Find the least common multiple of 20 and 30 to find the time at which they'll meet.
Multiples of 20: {20,40,60}
Multiples of 30: {30,60}
60 is the least common multiple of both 20 and 30.
The balls will meet again at
60 seconds.
Answer:
1/20
Step-by-step explanation:
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4 babies and 7 adults
B head=4x1=4 Aheads (3x7)=21 21+4=25 heads
B wings 4x2=8 A wings(4x7)=28 28+8=36