750 should be rounded up to 800. When rounding anything equal to or greater than 5 gets rounded up, while everything else gets rounded down. In this case the tens place value "5" is equal to or greater than 5, so the number rounds up.
It is 750.
700
The halfway point between 700 and 800 is (700 + 800) ÷ 2 = 750
745.00
It is the same distance from 700 to 800.
700 + 800 = 1500/2 = 750
750
750
700
800
800
800