Answer: 65/78 hours
Explanation:
You have to make 10 cases of brushes, it takes 26 hours for 4 cases so simple, you have the first number already, 26 for 4, how many for 8? 26+26=52 ok so to make 8 cases it takes 52 hours, now to make a single case it actually takes 6.5 hours so we could just go another 4 cases and they would have 2 extra brushes because 4,8,12 obviously so to do that it would take 52+26=78 hours, now for exact amount lets see, you have 52 hours already which is 8 cases, you need 10, so to that you would just need 2 brushes and for each case its 6.5 so 6.5+6.5=13 hours so then 52+13=65.
65 hours to make 10 cases of brushes!