Answer:
256.
Step-by-step explanation:
If a bacterium divides once in every half an hour, it means it will double in half an hour. Hence a number of the bacterium after half an hour will be 1×2=2. After one hour the number of bacterium will be 2×2=4. Similarly, it one and a half hours it will be 4×2=8. In two hours it will be 8×2=16 in two and half hours, it will be 16×2=32. In three hours it will be 32×2=64. In three and half hours it will be 64×2=128. Hence, in 4 (four) hours the number of bacterium will be 128×2=256.