How is 3.5 half of 7?

Half of 6 is 3. Half of 8 is 4. 7 is not an even number, so half of 7 will not be a round number. Half of 7 is halfway between half of 6 and 8-halfway between 3 and 4. So half of 7 is 3.5

Additionally, you could multiply 7 by .5 to divide it in half. Your answer will be 3.5, for the reason explained above.