The first time they beep together is LCM(50,48).
For LCM:
50 = 2 × 5 × 5 = 52 × 2
48 = 2 × 2 × 2 × 2 × 3 = 24 × 3
We can see that common factors are: 21 only
So, LCM = (21) × (23 × 3 × 52) = 1200 sec
Hence, they beep together after 1200 sec for the first time.