Answer:
t = 0.28 seconds
Step-by-step explanation:
Given that,
Force acting on a firework, F = 700 N
The momentum of the firework, p =200 kg-m/s
We need to find the time before it explodes. Ket the time be t. We know that, the rate of change of momentum is equal to external frce. So,

So, the required time is equal to 0.28 seconds.