Yes he is correct. This is because if you convert all the fractions to be out of 10, the equation is 8/10 minus 5/10 equals 3/10 which is correct. if you have any more questions let me know. i hope this helps :)
Answer:
6 000 because it is less than 6 500
It's B, and being mental won' help when having a calculator next to you) It's B for sure!!!