Under given circumstances, we have two ratios: 6:5 and 9:7.5. We can clearly see that 5 is 5/6 of 6. The question is: is 7.5 also 5/6 of 9? Let's check it:
9 * 5/6 =
= 45/6 =
= 7 3/6 =
= 7 1/2 =
= 7.5
It's correct. It means that the designer has to use a scale of 5/6 in comparison to the real dimensions.