A bike travels 4 miles in half an hour at what speed

8 miles per hour.

If he drives 4 miles in half an hour, it means that he will drive 8 miles in one hour. (multiply by 2) In order to drive 8 miles in an hour, you need to be driving 8 miles per hour. Therefore he traveled 4 miles in half an hour at the speed of 8 miles per hour.

8 miles per hour

because if it is moving at 4 miles every half hour that means you have to multiply it by two to get it in miles per hour and we all know that 4 times 2 is 8 so it would be 8 miles per hour = )