Answer:
The two watches will beep together for 20th time at 3:25 pm.
Explanation:
My watch beeps every 15 seconds and mom's watch beeps every 25 seconds.
Thus both the watches beep at the same time at an interval of 75 seconds.
75 is the smallest multiple of 15 and 25.
They beep together at 3 pm and when they beep together for the 20th time , we have to add 20 times the time taken for both the watches to beep together.
This time interval = 20
75 = 1500 seconds = 25 minutes.
The two watches will beep together for 20th time at 3:25 pm.