A bacterial chromosome has 4.6 million base pairs and one origin of replication…

Step 1. (a) Total speed: $2 \times 2000 = 4000$ base pairs a second. Time: $4.6 \times 10^6 \div 4000 = 1150$ s, which is $19.2$ min.

Step 2. (b) In $8$ hours, which is $28800$ s, each fork copies $50 \times 28800 = 1.44 \times 10^6$ base pairs. An origin has two forks, so it copies $2.88 \times 10^6$ base pairs.

Step 3. Origins: $6 \times 10^7 \div 2.88 \times 10^6 = 20.8$. We cannot use part of an origin, so we round up to 21.